Hot water pressure problem(bathroom)

The hot water pressure in the shower is low. Residents are complaining their water doesn't get hot. When the sink in the bathroom is turned on in the middle position then I get proper pressure in the shower. Even with the cold water shut off it does this. Any idea what is causing this? We've tried swapping mixing valves, checked for closed valves and can't seem to understand why the hot water pressure is low but increases when the sink is turned on. Seems to be an issue in multiple suites.