I have the tankless hot water heater and have hot water everywhere except in the shower. If I turn the shower off and just use the tub hot water faucet (have separate hot and cold faucets in tub) the hot water is hot. But the pressure is not great in shower head recently and hot water gets luke warm rapidly and I don't even use the cold. How can I fix this?

Are you saying that you have two valves one set controls the tub and the other the shower? What brand of shower valve, or valves do you have?