Actually, that is what can be read on the numerous Russian channels/forums today. (I began to monitor them after cruiser Moskva's sinking). According to Russian military experts and their fanbase, the reasons for the "difficulties" are:
1. Several stupid generals/admirals that just happened to be in the key positions. Some battalion commanders and ship captains were stupid as well. But the Russian army as a whole is very smart and heroic.
2. Opposition, "national traitors", plutocrats oligarchs - they undermine the mighty Russian Army from within. Putin was too kind to all that vermin.
3. NATO weaponry, recon, advisors, money.
4. NATO troops. Yes, they are there. Nobody saw them, but they are there of course because Ukrainians can not fight.
5. Clever strategy (no, I'm not kidding). The secret plan was to start light, to wear out Ukrainians, get NATO involved and deliver the final blow to NATO.
6. Because the Russian Army is civilised and does its best to minimise civilian casualties. Otherwise, it would "steamroll Ukraine several times already", as was recently announced by a Russian diplomat.