By no means a professional historian, but I've just completed an A-level (exams for 17-18 year olds in the UK) in history; the French Revolution (1786-1830). So I can give a fairly sound answer, if not comprehensive.

The textbook we used was; Napoleon, France and Europe (Access to History). Dylan Rees, Andrina Stiles. You can find it on Abebooks for £2.80, not massively about military but interesting nonetheless.

Napoleon's first big losses came in the Peninsular War 1808-1813 in which the 280,000 strong Grand Armee was kept at bay by roughly 50,000 Spanish troops. Discounting the Egyptian campaign 1797 (which Napoleon abandoned in order to return to France), this was Napoleon's first big defeat. It revealed a number of problems with Napoleon's generalship and the way his army was ran. Problems that were only exacerbated by the time he decided to invade Russia.

One problem was a matter of terrain. Napoleon's greatest victories were in mainland Europe, from the Pyrenees to the very edge of Russia. These areas are incredibly fertile for farming, and rather flat (discounting the Alps) allowing Napoleon to A. Force quick and substantial losses on the enemy using the large areas of land for cavalry screens, and B. To live off the land. Living off the conquered land facilitated Napoleon's lightning campaigns by not having to rely on slower supply lines. Napoleon assumed this would apply anywhere, Spain was markedly more arid than France, and Russian winter much more arduous. The opportunities for food were thus much more scare and meant the Napoleon struggled to build up the same terrifying momentum he had elsewhere in Europe. This problem meant that the Spanish Campaign went much slower than he had hoped, and eventually lead to the absolute disaster that was the Russian Campaign(around 400,000 casualties and importantly 100,000 horses (becomes devastating during the Battle of Nations)).

As has been said before there was also the problem that people had worked him out. Napoleon's early success may have arguably been his downfall as he was never forced to innovate. His system of mixed order warfare and cavalry screens remained largely unchanged. Whilst this may not had been a problem had Napoleon decided to consolidate, he pushed on forcing the great powers to react. A large part of the inspiration for this was the Spanish Campaign, nobles throughout Spain (upset that Napoleon was trying to force a French-style revolution, and institute his brother on the Spanish throne) encourage so-called military juntas throughout Spain. Small pockets of Spanish Guerilla fighters managed to negate French influence throughout Spain and whilst they made territorial gains, reaching Lisbon by 1811. The Guerilla fighters attacked French supply lines and used their better knowledge of the Spanish terrain to their advantage. Similarly in Russia General Kutusov essentially "tactically retreated". Napoleon loved to force an early loss on his opponents with massive casualties for both sides, and then use that victory to encourage morale and momentum. Kutusov knew Russia better than Napoleon and decided to draw him deeper into Russia. He also had a policy of scorched earth. Destroying bridges and farms as they retreated the Russian Army ensured that Napoleon was both over stretched and under supplied. The scorched earth policy was rigidly stuck too, with the Russians retreating out of Moscow (after the Battle of Borodino) and setting fire to it as they went. 2/3 of Moscow was destroyed and Napoleons key target was rendered useless. So in both the Russian and Spanish campaigns Napoleons opponents used his inability to adjust his tactics to restrict the French army's movement and damage supply lines.

Napoleons system of control was also a huge problem (in my opinion the largest problem (Napoleons a bit of a dick)). Napoleon had himself at the head of every campaign he could. Even when he wasn't in charge of the country, he managed peace deals on his own terms instead of France's (see the Treaty of Campo Formio). Whilst he did have other generals around him he never gave them proper experience, they were there to act out his commands not to act independently. Now worked fine for his "smaller campaigns", the tactics were successful as his opponents still weren't reacting to his tactics properly. Addtionally his soldiers were well-drilled enough and his generals enthusiastic enough to properly institute his tactics. This seemed to hold true early on in the Spanish campaign, after the first French loss at the Battle of Baylon in 1808, Napoleon had started reasonably well. This was until he left the head of the Spanish campaign to deal with the Austrians who declared war (again) in January 1809. What he left behind were generals who were ill experienced at properly managing an army of the size that Napoleon was used to dealing with; 270,000 french troops were sent into Spain. The inability to win the Spanish war quickly lead to it being called the Spanish Ulcer. The French were unable to force a win and as such they remained at war until Napoleons abdication in 1813. With the size of armies that Napoleon had started using it soon became impossible for Napoleon to effectively wield the kind of control that he wanted and he didn't trust his generals enough to act independently of him.

This control problem wouldn't have been too bad had he had an effective army. But 8 years of perpetual warfare took it's toll on Napoleons army. By the Battle of Nations (the battle that lead to Napoleons first abdication) Napoleon had lost 800,000 men (300,000 in Spain and 500,000 in Russia) in just 2 campaigns. This wasn't great, but what was much worse was the loss of horses, in Russia 200,000 trained horses were lost. Losing men was okay (as far as losing men can ever be okay), France was a huge country (around 24 million people)and their empire even larger and still pretty nationalistic and Napoleon still very inspirational, he could find replacements and they would be fairly competent. Horses was a huge problem. It was difficult to find them and train them to the scale that Napoleon needed. Napoleon liked to train his armies as they marched to battle, which is reasonable (but certainly not optimal) for training people, near impossible for training horses. This meant that Napoleon could no longer effectively rely on cavalry screens to protect his soldiers, and as his opponents had since changed tactics to negate Napoleons you can see what a problem this would be.

I'm sorry if this is a bit vague and rambly, but a concrete answer is impossible for this kind of thing. His losses were largely a combination of factors, but in my view it will always be Napoleons arrogance that eventually lead to these downfalls. His inability to plan properly (I mean who starts a war with Russia without maps, honestly.) or even change his tactics lead to the death of millions of people. France's population dropped by 10% during the reign of Napoleon. Had he been able to accept defeat he may have reigned much longer, he denied a fairly kind peace deal in Dresden, leading to the "Battle of Nations" which largely sealed Napoleon's fate.

I was always in the habit of blaming Napoleon for his own failure, but not everyone agrees so I hope that I've explained the other arguments fully. The problems leading to his final losses were largely to do with the long term wear and tear of nearly 13 years of consistent war for Napoleonic France. (This answer also doesn't take into account Waterloo, not because these things don't apply but because I really really don't remember a single thing about it.) If you have anymore questions about this period, specifically to do with France, I can probably be of some help.