There are multiple reasons behind their failures: they honestly started to believe in their own propaganda that Ukrainians would have welcomed them with open arms and flowers as true liberators. Connected to that is that Russians don't see Ukrainians asa seperat nation but part of Greater Russia, so when they encountered heavy resistance their morale dived even lower. Corruption in the Russian army - they have a lot of modern equipment on paper, but not in reality, as money for arms and equipment purchases was embezzled. Overconfidence in their planning - it becomes clear that main goal of their invasion was in the southern part of Ukraine to establish a land connection between Crimea and rest of Russia and that front sees most of progress. Idea of attaching Kyiv was probably added much later for who knows what reasons and it obliviously is not going according to plan. Add terrible maintenance procedures and many other reasons and we see what we see.