Why wont my gas wall heaters main burner ignite?

I have a Cozy 50,000 Btu wall heater. The pilot light stays lit but the main burner will not ignite. The thermastat has been checked and it is okay. Is it possible that there is not enough gas pressure to the unit? Any ideas? I've had two technicians come out already and neither have fixed the problem. I cant afford to keep paying for nothing.