I own a two story 3 year old house. My downstairs shower and upstairs shower is not getting any hot water running through it. Our upstairs master bath gets plenty. They have worked in the past but rarely get used and when we had guests over we realized the hot water was not coming through them. What could this be and is it something I can fix? Thanks

Are these line exposed to the cold and you might not be aware of it? Heater apparently works. Hot water pipes also cool and freeze quicker than warm. If it was fine before and if its cold now the pipes are in a very cold long enough spot to keep cooling pipes.
Since its both showers this rules out the valves unless it's the anti scald on each fixture that needs adjusted to compensate for the cold pipes but worked before for hot water
Signed 21 Boat

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Cold water comes out warm

Cold water works fine, but when I turn the lever all the way to the left for hot water it barely trickles out

Cold water works fine, but when I turn the lever all the way to the left for hot water it barely trickles out

Shut the water off and open up the valve. Remove the cartridge and check and clean the inlet ports on it. Before you button up turn the water back on to flush the supplies and open valve body. You should have a strong stream out of the valve when you do this. If not tell me about it. Good luck, Tom