After usage of hot water, blower comes on, ignitor ignites but no gas released to heat hot water. Why? This is a Rheem Power Vent.

Does the flame come on even for a few seconds?

I agree with the warranty statement. Go there first. Mine was out of warranty. I had the same issue.

Don't do any work on this yourself unless you are qualified.

What I recommend - first check your vacuum switch. You can short the wires on your vacuum switch. All you need is a wire jumper or paper clip for this test. Turn the power off. Remove the leads to the vacuum switch and short the leads. Turn the power back on. If the gas works then, you may have a faulty switch or a plugged vacuum line or water in your line. If a plugged line, you can actually buy new line and run from the motor to your switch on the outside of the tank. Rheem recommends this.

If it is not the vacuum switch there is something else to check. On my unit, I found a loose male terminal. It was the red, plastic covered terminal on the gas valve. It is the hot line from the 24V transformer. I simply closed the gap with a pair of pliers and this fixed my problem. To test for this you can restart your heater with the panel open. Once the igniter is hot, you will hear a click from the controller - this is switching power to the gas valve. When you hear this click, move the red wire on the terminal on the gas valve to see if the gas comes on. You can also check the voltage at the gas valve on the two terminal screws. You should be getting 24 volts AC. If you are getting 24 volts at the terminals on the gas valve - probing the screws - and your gas does not come on - you may have a valve problem. You may want to check other terminal connections on the controller and vacuum switch to be sure they are not loose.

For me, I also had to change out the controller because it would only provide 24 volts to the gas valve for 1 second then drop to zero. I found a controller on EBAY for $73.00 vs. $ 150.00 from other sources.