Sounds like you don't have enough bellows. 15-1/2" is pretty short for an 8x10" camera. Most have at least 24".

Is the bellows itself maxed out at 15.5" or is it the focusing track? If it's the bellows, there's not much that can be done, unless you want to build an extension lensboard or back or replace the bellows. If it's the focusing track, then it may be that there was an extension rail made for this camera at one time, and you could look around for one at places like, or you could try to build a new one yourself.