All newer shower valves have a means to set the maximum water temperature. This could be a cam, a screw stop, or possibly some other means. This results in limiting the amount of hot water that can get to the showerhead. Then, there's also in many a spool valve that is the guts of a pressure balance antiscald feature. This spool valve could be stuck so it is restricting the hot inlet. You may need to take it out and clean, or replace to free it up (if you had it out and shook it, you'd feel and probably hear the spool move from one end to the other). Without knowing the brand and model, it's impossible to say for certain.