I had the same issue with an M6 TTL and 35mm Summicron ASPH. The best explanation I could find was that the flanges for each were at opposite, extreme ends of their tolerance ranges.
I sent the then newly purchased camera body and lens back to the dealer, who swore they couldn't find an issue and that the two mated perfectly.
When they were returned, I found that they had the same tightness as when I'd sent them off. After getting a few pointers from other people, I summoned the nerve to push the lens past what had felt applying like too much force. The lens then snapped cleanly into place. It took removing and mounting the lens a few times, but it now mounts without concern after this "loosening excercise".
While I can't say with certainty that the same treatment will work without ill effects on your camera and lens, my experience was a success.
