I'm having issues getting hot water out of two showers in the upstairs of my house. The weird part is that both sinks in both bathrooms put out hot water. The water in the showers come out with enough pressure however when turned all the way on the water only gets luke warm, and barely luke wark, just warm enough to not freeze when you get in. But it does heat up, so SOME hot water is getting to the shower head. This is the case for both showers. I have a shower downstairs that puts out hot water fine. The knob for the upstair shower is a single knob, it rotates about 90° to go from off to all the way on, which should be hot. Any ideas??

would you happen to have a moen posi-temp faucet in your shower? if so start by replace the cartage.
if not a moen. still most likely the cartage.

i would check for a limit-stop before changing parts.
limit-stop is a non-scald part that keeps the handle from rotating too far into the hot zone. it is totally adjustable. can't say more as different companies accomplish this different ways..

Most likely the high limit needs to be turned up a bit. It usually involves removing the handle and rotating a stop or turning a set screw.