Pilot flame lights off with re-ignitor and stays lit?????
Then when you turn control knob to 'ON' position main flame will not light?
First thing......is pilot flame engulfing the tip of thermocouple?
Thermocouple has to get hot in order to generate milivolt signal which holds magnet inside gas control so propane can flow.
If not getting hot enough no gas can flow.
Bad thermocouple, dirty thermocouple, loose connection between thermocouple and gas control (should be finger tight then 1/4 turn) can cause lack of main flame.
Dirty burner tube, plugged main orifice can disrupt propane flow. Orifice should be cleaned by soaking in alcohol and then blow out with compressed air........it is a metered orifice and you should not be sticking metal objects thru it to clean.
And last.........gas control could be defective
That is if LP Regulator is supplying 11" WC of propane pressure
Does main flame light at all?
ON Edit:
Just read your post.
Having to tap on gas control means it is time for a new gas control.
They are not 'serviceable'
It is sticking closed which means it can stick open.
After 15 yrs. you have gotten good service....get a new gas control