My rokinon 8mm is definitely on the stiff side, but after the first few times it's reached a "this doesn't make me nervous" level of stiffness No idea why it started out stiff, or why it got better. I didn't feel any grinding, so I'm guessing it didn't lose any metal.
I have noticed that it's a lot harder to do when I'm outside in hot weather and it's been sitting there soaking up heat for a while. Not sure what the temps are there -- try popping an ice pack on the mount part of the lens for a few minutes and put it on? If that lets you turn it to locking and take it off again, at least you know it's capable of doing so... and maybe twisting it there a few times will help make it fit better in the future. Not sure.