Reverse Problems

Hello,
I picked up a 1963 Buick Skylark and it runs great but once in a while the reverse doesnt want to work. You can hear it shift into place but it doesnt go anywhere. Can some one please tell me what it might be or is it just the transmission needs to be rebuilt before it goes out?
Thanks,
Brandyn

 

Maybe some info about what kind of transmission you have would be helpful.

 

Its an amuminim V-8 Engine with a auto transmission