Your water heater should get much hotter than that, and it probably is. Most of the time, when water is warm, but not hot, it's because it's getting mixed with cold water somewhere in the lines. How does this happen? It happens to a lot of us until we learn.
Check all your faucet handles and make sure they are turned off, especially showers and outside showers where hot and cold come out together.
If you have an outside shower and have the shower head turned off, but the faucet handles till on, water will mix hot and cold and feed back luke warm through your system, making everything just warm, never hot.
This is the first thing to check. Check ALL faucets and make sure ALL are off, not just off at a shower head. 99.9% of the time .... THIS is the problem.