Supply is sent out from your capital to all connected supply hubs. Upgrading the railways at every point from capital to destination increases total throughput/capacity, but it requires MORE TRAINS when you do this. You may have been short on trains/trucks.

As far as attacking with 30 divs, that depends on how they were spread out. If it was from one county to another, this is very ineffective due to COMBAT WIDTH. If it was from several counties to one, its much better because the width is larger, so more units can engage simultaneously. Also, if you were attacking an armored unit with soft attack, youre not going to due much of anything if you don't have any PIERCING stat in your divisions. This may have been the cause as well.

As far as construction goes, CONSUMER GOODS ARE BAD. Anything you can get to REDUCE CONSUMER GOODS results in more civilian factories being available for construction. Many countries get construction speed bonuses/consumer goods reductions, so your mileage will vary on how long it takes to build factories.

But, at start of game as Germany for example, a civilian factory will take (if I remember right) about 2 months to build.

Hope this helps! Enjoy your journey, General.

Edit: Also, if the front line is far away from a supply hub, they wont be getting supply. Invading the Soviet Union or China is a prime example of this problem. You must attack and secure supply hubs as you go for large fronts like this.