Indeed the mount is bent. It only takes a couple of degrees to bind.
Shutterfingers check is the way to go, but if the lens otherwise works; the bent section
can be straightened (DUCK) with a large screwdriver twisted between the offending part and surface of the mount.

Be aware! You will find some later responses to this statement using the following terms. :o)

SACRILEGE..HEATHEN..BARBARIAN..BRUTE! ...............You'll ruin the alignment/focus/value/etc of the lens.