DD, do you think it's possible iron build up?? Don't know how old your home is, but a lot of times in older homes ive worked on in the past, fixtures that aren't used often iron will collect and you can't even turn a knob.
If this happens to be your case, start by flushing it with vinegar, second option is acid (muratic). It'll change the form of the iron from solid (disolve) long enough to flush out the fixtures, valves, etc.
it's a pain, but when your working on antiques that are priceless to the owner.. That's what you gotta do. In your case, it would be easier to flush it out, if its a build up, than tear into a "back to back" shower project to replace.