It seems that Russian forces have been dogged by a host of factors which prevent them from ending this war quickly. These include the "rasputitsa," or muddy season, which restricts practically all vehicles to roads; low morale and poor training among some of their troops; poor planning and logistics; unexpected resistance from both Ukrainian soldiers and civilians; and more. Here's a viddy which enumerates these and provides a link to another site which details vehicular and air losses on both sides:
https://www.youtube.com/watch?v=SVD-nBvSBkw