Some are thinking we're still in the softening up phase and the real push hasn't started yet.
I don't know since I don't have access to Russian force deployments.
I also don't have access to Russian Force Deployments, but that excuse rings hollow.
First, it's not russian doctrine. They run their armor in fast and run the Motor-Rifle troops in afterwards to hold land. Which is what we saw with T-90s running in first under air cover, and BMPs and BTRs coming in later. That's why most of the videos taken by civilians on the web are of APCs. That's what comes in after the initial push. So the Red Army appears to be following their doctrine.
Second if you were planning a softening up stage followed by a hard push, your softening forces would have enough fuel and ammo to hold until the follow on force, and there are reports of Russians running out of both.
Third, that's a dumb strategy in modern warfare. You don't send a force against prepared and warned defenders unless you think you can beat them. Sending an initial attack followed by a larger wave as the plan invites defeat in detail.
It's more likely IMO that the Russians sent what they thought they needed, and it failed. Now they are committing whatever reserves they have and trying to save face by saying this was the plan all along.