I have a couple of mounts like this. One of my Eotech clones (a G&G one) just WILL NOT tighten onto a rail as hard as I try.
I do have a Spectre DR that has seen some heavy use to the point that the little plate that grabs onto the rail has become bent out of shape and also won't grip. Mind you, that weighs a ton so I'm not entirely bothered!
Edited for brevity.