What could cause low hot water pressure only in the line to one shower?
I've had exactly this issue, on the hot line to a tub, and eventually (disassembling the piping as I went) found an elbow almost plugged with solder - a half-inch pipe with a 1/16" or smaller hole through the nearly-plugged fitting. Someone got over-entusiastic feeding solder (probably 40 years ago for the one I found - and someone lived with the tub that way for probably 40 years, which is the puzzling part.)