If you're willing to spend a little time, and are comfortable using basic circuit tools, check the following:
(1) Thermostat calls for heat and starts the heater.
(2) 12vdc is sent to Fan relay through the normally closed Heat Limit Switch.
(3) There is a 10 second delay then the Fan relay sends power to the fan motor.
(4) Fan air flow closes the sail switch, which sends power to the gas valve solenoid and air temperature limit switch.
(5) The ignition system sparks and lights the pilot and main burner.
If you have to rap the unit with a hammer, one of the solenoids or safety devices might be sticking. Could be dangerous to allow to run if a safety device is not working right! It seems likely that if the blower is running the gas solenoid valve might not be opening, or the sail switch isn't operating.
You'll have to do some detective work to determine if the problem is with the wiring, Heat Limit Switch, Fan Switch, or thermostat..
If you are comfortable, you can use a multimeter to follow the progression of the voltage, observe the working during start up, and part-by-part, find the failure(s).
Hope this is helpful.