Sometimes it helps to compress the back against the body (where it opens) while holding the rewind release and pulling up on the handle. Also, there is a possibility that the film cartridge has rotated and/or is putting pressure on the inside of the back and preventing the release from activating. Or there just may be grunge/piece of film in the 'receptacle' where the film cartridge resides creating pressure on the back.

I just had light seals installed in my F2 and it created enough pressure on the release mechanism that I have to squeeze the back/body together to get it to release properly.