Not enough troops. Ukraine today has over 700,000 men in uniform and the advantage of well prepared defensive lines. The Russians would not faire any better.Alright get your predictions in for this winter.
With up to 500,000 troops, what would you do? Me I will go for Kiev Offensive 2.0 from Belarus direction. Back at the start of the year they weren't enough troops from that direction and the crappy road from there during mud season meant supply lines could not support a major offensive. With winter and the ground frozen it may be possible for a decisive attack.