I've had that problem on a different brand of riser. It sucks to have your light slightly to an angle, I just learned to live with it. If you're like me, you hate helmet mounts (for commuting, not mtbing) and just want to keep the light on the bike. If your mounts are getting loose, cut the rubber strips that come with the mounts so you can fit them between the mount and the bar. This should stop any sliding.