Look at the rear of your water heater (you may need to remove a blocking panel screwed to a cabinet floor to see it).
There are several valve scenarios that manufacturers use to bypass the water heater when winterizing. One way uses three valves and others use a one way check valve in the discharge line (with one or two visible valves).
Since you are getting hot water out your hot water heater's discharge line I can eliminate the one valve system as you problem.
You most likely have the 3 valve system (similar to the picture)
Make sure the cross over valve (called the bypass valve is closed (cross wise) when in IN and OUT valves are open (IN LINE). If the bypass valve is open, cold water will be mixed with the hot water and what you get will be barely warm, not hot. Make sure your valves look like the photo.
If you are OK there, then look at your shower wands (inside and outside). If the shower valves are open at those faucets and the cross button is closed on the wand, cold water can cross over into the hot pipes when the hot water is used causing the same effect as an open valve.
If neither of those is your problem (one should be), you will need to check each faucet that has a "one handle" mixing faucet for a leaking internal seal.
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