lopek77: If it used to work and doesn't any more I'd carefully examine the clamp for cracking or deformation; a sudden failure could have unpleasant consequences. Be certain that there is a gap at the clamp opening so that the screw is applying tension to clamp the seat tube and not just closing the clamp on itself. You might also try cleaning and greasing the QR threads and cam so that you can easily adjust them. The clamp lever should rotate closed smoothly with steadily increasing pressure. You should adjust the clamp screw so that it takes considerable pressure to close the lever; it should leave a mark on your palm if it is tight enough.
Edit: I see that FB beat me to it while I was typing.