Why isn't Russia using it's full power on Ukraine?

Different styles of warfare. Putin mobilised 10% of his army for a short fast ground invasion, expecting little resistance.

What he actually got was a lot of resistance. He prepped no on other than his closest circle as to his intentions, and there weren’t fuel or food supplies to support an invasion going longer than 2 weeks.

Then there’s the economical warfare taking place via sanctions that have hampered his advance even more.

Essentially, a lot of things that Putin underestimated which have weakened his position.