Hot water running out too quickly
I have a Rheem mod# 21V40-7 gas fired 40 gallon water heater. The hot water seems to run out too quickly for the size of the heater, which is particularly noticeable in the showers. I find myself constantly adjusting the temp in the shower toward hot while I'm in there in order to keep it from going luke warm. I noticed this problem first myself, and now have the same complaint from tenants now living there, without prompting them.
I first checked for anti-scald devices in the shower valves, which I removed for troubleshooting. I also raised the temp on the hot water heater itself to just below the max. Neither step seemed to help the problem significantly.
Any suggestions on cause or further troubleshooting would be greatly appreciated.