Reply:
Do you get adequate HW anywhere else in the house?
If yes, then there is a problem with the shower faucet itself. If it is a single handle type faucet, the faucet or the cartridge could need adjustment or replacement.
If you do not have good HW anywhere in the house, we cannot advise you on what to do because we don't know the source of your HW.
Good Luck. e-plumber