Hot water in one but not both Showers

We have two baths on the second floor. We have an oil fired boiler. Water pressure is not a problem in either. One bath gets adquate hot water. It has a single lever delata faucet with scald protection. The other bath gets hot water only for a few seconds then occasionally, it will get lukewarm at best. It has separate controls for hot and cold water and no scald protection. What could cause this?

A clogged inlet screen filter; a bad washer; dirt or something clogging the hot line that gets pushed up against something blocking flow, then when it is turned off, it falls back. You might have to take the stem out - run some water to blast it clear, then reassembly. One of the pros might have some other suggestions.

Jadnashua, thanks for your response. But this has been a problem for a long time and we have changed faucets.

Are these back-to-back? Some valves are known to present a cross-over, meaning that they mix the hot and cold water for things downstream of them. try turning the knob all the way one way at the other shower. It could be the vanity faucet, too. If you shut the valve to the WH and run hot from each of the faucets, you shouldn't get any flow (if you can turn it all the way to hot). See if you do...if so, you've got a worn or defective valve somewhere that's providing a cross-over. Fix that, and you should be okay. It still could be something stuck in the pipe that with the flow, moves to block things. Could be a chunk of solder, calcium, teflon tape, all sorts of things to include a pebble.

If you replaced the faucet and still have the problem, it may be that the hot water shut-off valve for that faucet is clogged with debris.