Rheem heater works one time and then won't light
The heater is about 20 years old. We checked it and weren't getting 24 volts for the gas valve. The ignitor control was replaced with a Robertshaw 780-910 Hot Surface Ignition Module and it worked fine. The next day, it wouldn't light again.
We reset the control by turning off the power and it worked perfectly. A few hours later, it wouldn't light up again. Over the next couple days, the same scenario. It won't light. I unplug the heater and plug it back in and it works until the heat goes off and tries to come back on.
What should we check? I noticed the control is supposed to have either a 4 or 7 second ignition trial time, but I don't know how to change it to a longer time or that this would help.