Interesting problem….can get hot water out of bathtub/shower faucet if taking a bath…have water pressure out of shower head…just water that is not hot. Any ideas about the problem?
This sounds as though the diverter valve is not opening properly when you switch from tub to shower mode. Or the anti-scald adjustment needs to be changed.
How exactly are you doing this switching from tub to shower mode? Are you turning a valve on the wall to change it from tub to shower? Or are you simply pulling up or down on the lever attached to the tub spout?
How old is the faucet? Best guess will do. Please describe what you have. Brand?
Single faucet that rotates to increase temp? Two faucets, or three?
My thought on this without knowing much about what you have is that the anti-scald protection within the faucet needs to be adjusted. Although the water may appear to be hotter coming out of the tub then it does when it exits the shower head. But because the water is spraying out of the shower that alone can make the water feel cooler then the heavy stream exiting out of the shower head.
The water temp can be adjusted, but it must be done with care, The whole idea of anti-scald is just that. But fine adjustments can be made.