1) Whats the temp of your incoming cold water?
2) Whats the temp of the water your showering with?
3) How many gallons per minute comes out of your showerhead while your showering?
Use a 5 gal bucket to see how many gallons per minute your showerhead flows.
Give me that info and I will tell you if the heater is operating within its limits. If not then we will go into the possible problems.