Are you running your heater on gas or propane or both?
Does it fire up on gas and run for 15-20 minutes and then turn off, meaning the water in the tank is now hot? Or does the gas burn for just a moment and then shut off.
Need a little more detail what your experiencing.
Need to determine if the water is mixing cold and hot (like at a shower or an outside shower or faucet, or if the heating element ... gas or electric ... is only running for a few moments and then shutting down.) This would give you a little bit of hot water, but would quickly run out.
And, when it starts running cold, does the water heater turn on again to heat up?
Help explain a bit more so we can help you.