Originally Posted by Velognome

+1 on bottoming out. Make a couple passes with a file on the flats, this ensure that clamp is pulling tight, to bottoming out and without the need to over torque the bolt which will inevitable end with if failing in the middle of ride on the hottest day of the year.

But, if the clamp has been slipping, it's probably toast. The lands are most likely worn from the friction and won't make a good purchase anymore.