I had a very tough time getting my bolt stop pin back in also, the first time. I have a Mark II and the Frame/Receiver fit is quite tight. I think the problem was that I wasn't getting the bolt stop holes on the frame and the receiver back in the correct alignment.
Try giving the back of the frame a whack with a rawhide or rubber mallet to get the holes ligned up correctly.
After I got it in the first time, I never had a problem again.