We have a problem with two of our upstairs bathroom tub/showers. When you turn the dial to the warm/hot position the water flow goes to a trickle. Cold water is at the regular flow rate. These two bathroom tub/showers are hardly used. Cannot say the last time hot water actually ran through them. Probably a couple of years. I know now that they should at least be turned on to let the hot water run through on occasion but that does not help us now. There are closets on the opposite sides of the fixtures but no valves to turn on or off. Any suggestions?

What brand and model of faucet, as in Moen? Delta? Other? Sounds like dirt in the hot section of the valve. Most likely you will have to tear the faucets apart, flush them out and rebuild them.