Robot slips from slope only when facing down the slope

edit

I am experiencing a curious problem when driving my differential robot on a slope surface. No matter if I assign to both the robot wheels and the surface the friction of 0.9 or 100, the robot slips uncontrollably down the slope, but only when it is facing down the slope. It can stay on the slope without any slip if facing up the slope or to the 90 deg.

My robot is a simple differential robot with two caster wheels on a freely rotating suspensions up front.

Does anybody observed similar problem? Any idea why is this happening and how to fix it?