How do I fix my shower?

I have two bathrooms, each with a tub/shower combo. In one bathroom, both work fine. In the other bathroom, the water out of the faucet for the bathtub gets great pressure and gets very hot, however, when the water is directed to the showerhead, it only comes out lukewarm, even if it is scalding hot in the bathtub faucet. What is causing this drop in temperature from the tub to the shower level?