I would suggest having someone look at it. Do you have a camera store nearby? There are a few possibilities that might be causing the problem. The lens may have over-rotated when mounted, the release pin might not be working, or a screw on the lens might be loose and sticking out slightly.
In the event it's the release pin, do you feel the usual spring action under your finger when you press it? For the other two options, I'm not sure you can tell if one of them is the cause. Is there any slight wiggle to the lens when you try to take it off?