When I turn the kitchen faucet on for hot water - sometime hot water comes out sometimes just warm water. The same problem exists in our guest bathroom. Just never know if hot water will flow. The problem is random however most of the time it not hot water that flows. Our master bath faucets seems to work ok even when the kitchen faucets do not.
I would suggest that you check the hot and cold water valves or cartridges. Could be an o-ring or seat that is not sealing properly and allowing cold water to mix with the hot.
If you have an automatic washer you may want to turn off the manual valves at the hose connection, just for a test. Sometimes the water will flow through the solenoid valves even when the washer is off allowing hot and cold water to mix yielding just what you have happening. Also check the cartridges as Jeff Cates suggested.