I agree it is probably just a shrunken or nicked o ring.
Take the stem to a plumbing supply house, they should be able to size and replace the o rings.
Or find you a replacement stem.
Clean out any minerals and crud inside the brass valve in the wall, rinse with plain water.
Put a thin layer of plumbers’ silicone grease on the new o rings, and a little on the threaded brass areas also.