Armstrong Ultra sx 80, pilot light lit, but heater doesn't turn on

Try cleaning the sensor that's the rod in the flame if that doesn't work you may need a tech to look at it .

If it has a thermo-couple, try replacing that, if it does not have one, I have no clue.

Is this a standing pilot?
If the pilot remains lit then your thermocouple is probably fine. If it were bad then your unit wouldn't hold a pilot. Is the valve in the "ON" position?

Is there power (24 volts) going to the gas valve at the time it should be opening up to send gas to the burners ?