Well, if you dont show us some code or at least describe it i cant see a way to help you.

Anyway, i can tell that when i upgraded from 6.3 to 6.4 i noticed some problems with the physics engine behavior but that was because there was changes on the physics engine itself i believe, like the fact the physics entities movement were no more frame dependent, etc.

I noticed i needed to change the PE entities properties like friction, damping, elasticity, because they slide too much, its a matter of testing.