shut off water supply and take valve apart. I'm guessing you have an
anti-scald valve, which has little pinholes to let water in.

When putting in new plumbing, a plumber should flush lines before
putting the valve in, to allow pieces of solder to be flushed out, so
they don't get hung up in the valve.

If you haven't used this shower valve since the house was built it's
possible that there is a piece of solder clogging the valve. Probably
just needs cleaning.

The other thing is, if you're on well water, you might have sediment
coming into the lines, causing the same problem. After you clean the
valve, consider putting in a whole house filter to avoid future
problems.