Slow hot water to shower faucet and head?

Here's one that a plumber couldn't fix...at least he never came back. The hot water, not the cold, in our 2nd floor shower, both faucet and head, comes out very slow, like showering under a sprinkling can. No leaks, copper tubing, shower head filter clean and water saver out, new boiler, and pressure was increased. The sink, which feeds from the same tubing, flows fine. The same feed line feeds the kitchen and powder room and their fine. Valve was checked. What's the answer? frank