Have a Rheem 90 plus propane fired furnace. The inducer motor starts, igniter initiates, the burner does not fire. There is a short delay then igniter comes on again but burner does not light. What could be causing it.

Make sure the gas valve is set to "on". If the gas & gas valve are turned on, there should be 2 wires going to the gas valve. Monitor across those 2 terminals for 24 volts AC from the time the ignitor comes on until it goes off. If you get 24 volts, the gas valve is likely bad. A double check is to use a BBQ lighter near the ignitor, again from the time the ignitor starts to glow until it shuts off.