We recently moved into a house that has two bathrooms, both with single handle faucets, and one flows nothing but cold water while the other is the opposite. I can rotate the faucets the complete rotation from cold to hot, and the adjacent sinks in both bathrooms get hot and cold water.
So you are saying the hot/cold works on the sink faucets, but not in the showers. Hopefully hot/cold water has been properly plumbed to the shower faucets. Has there been any renovation work done on these showers recently?
Personally, I think it may be a worn out washer (or some doohickey) within the shower faucet preventing the mixing of hot/cold. Post some more info on the brand and perhaps even a picture of the faucets.
I'm sure the experts will chime in with solutions.
A DIY Noob that knows just enough to be dangerous.
What kind of valves - Symmons, Moen, other? You most likely just need to replace the cartridges/stems. This is not unusual for pressure-balanced valves that haven't seen any usage in a long time. Maybe the house was unoccupied for several months?