If it only heats to luke warm then there are two suspects (3 depending on the water heater model)
1: The thermostat.. There is a piece of foam black with lettering T-Stat ECO
The T-Stat is behind the foam,
2: Somewhere you have some valves set wrong
2A: Most common is what I call the DOG SHOWER, the outside shower near the dump valves used to rinse things off out there.
2B: If you have a 3-valve bypass system.. The bypass valve.
2C: The indoor shower
And on the model that has a 3rd option, (The 10 gallon heater in a six gallon hat) the mixing valve.
Only possible options.
My money is on the valves.