Tamron Lens mount issue.

So my girlfriend has a tamron 75-300 that she told me would no longer mount to her camera. It would turn a tiny bit then stop and you couldnt turn it anymore. So I took a look at it and what she said seemed to be true, I tried mounting it to her Rebel and then to my 50D and had the same result..it wouldn't turn. Upon inspection there didnt appear to be anything wrong with the lens. Then I decided to try it again, this time with a bit of effort, I had to kind of force it but I got it to go on. Once it was on the camera I tried it and it worked fine. Then I thought it was stuck on cause I had the same problem getting it off, it was very difficult at first.

I looked at both the camera and lens mounts again and cannot find anything wrong with either. I took the lens on and off of the camera several more times, and it seemed like after doing it a few times it got easier and I didn't have to use nearly as much force (which is good cause I was worried about damaging something by forcing it.)

So at this point it goes on and off and works but it is still harder to do than it should be. There really is no point of sending it in since it is cheap lens being used with an old Rebel so spending any money doesn't make sense. My question is, has anyone else had this type of problem before and/or does anyone have any ideas of what the cause of the problem could be?

Thanks

Nick