Obba, RuAF moved their best to Kherson where the battle is fierce and moves slowly. Because those best parts are south, the north/east is poorly defended (and btw the mobilized troops on their way to the front are even worse for they have no training and no battle experience), hence Kharkiv success.
But you can push only so far without resupply and preferably rotation. We have no idea how much fresh troops Ukraine has i reserve to keep the pressure on.
And, something IS going on as we write in the eastern front.