Could the answer be, that he does not want to invade Ukraine? If you look at it that way, he has done everything humanly possible to avoid invading Ukraine and still safe face and be seen at home as protecting Russians and Russia's interests. But the US and the Ukraine government are making it absolutely impossible for him, no ceasefire, just sending in more and more troops, killing civilians, aiming for total unconditional victory as the only possibility. That gives Putin no way out. There is absolutely nothing he can do to come out of this without looking like a big loser. So if this is going to continue, eventually he will have no other option than to invade Donbas. I just think it is very bad policy to try to corner the Russians. They are extremely dangerous when cornered.
I'd like you to understand, I am not on the Russian side, my own country was invaded and occupied by them and I don't have any reason to like them doing it to someone else, I am just trying to explain how it looks from their side.
Ukraine is absolutely nothing to want to deal with. If Ukraine joins EU, we will have a new Black Friday and the world economy would go in tail spin. If Russia took them, it would bankrupt the country just as well. It is 50 million of impoverished people with almost no industry, limited natural resources and the only thing going for them is their agriculture, which is bad news for EU. EU has quotas on agricultural production and has a huge oversupply of food. It is the hottest political issue in EU just after the federalization attempts. Ukraine is the size and population of Greece, Portugal, Romania and Bulgaria combined and their economy is worse off than either of those countries four poorest EU countries. Nobody in their sound mind wants Ukraine.