Why isn't there hot water coming from my faucet after stem replacement?
This sounds like an older style (washer type) installation. Is it possible that part of an old washer is stuck in the seat, or was pushed back behind the seat into the pipe, preventing water flow? First, make sure you have hot water TO the tub area. Turn the water off and remove the hot side, stem and all. Inspect for obstructions. You may need to briefly turn the water back on with the stem OUT, to blow out any obstructions. Have a helper turn the water on and back off, as you watch for any debris coming out. Once you have established hot water to the valve, you can check for an obstruction AFTER the hot water valve where it feeds into the tub spout, either part of an old washer or other debris. With the valve stem out, and the cold water valve off, try blowing air back through the spout to clear this (it will go into the hot water valve area) and then clear the valve out again before replacing the stem and washer, handles, etc.

Nine times out of ten, some obstruction will be in the line either before the valve or after it, where it feeds into the common pipe to the spout.