Problem with joint using NX_BF_FROZEN_ROT

Hi I am making a pinball game using PhysX without NxOgre, I have my paddles set up as joints and when they are in their top position I use the bodyflag NX_BF_FROZEN_ROT to freeze it until I realese the button (else it will jitter). The problem is when the ball hits the paddle and it´s frozen the ball "climbs" over the paddle there seems to be some force effecting the ball. If I hit the paddle from behind with the ball it gets a push aswell. Anyone know what could be causing this?

Best Regards Andreas