If you have turned your control knob to 'on', have maintained the fireplace and the gas is not partially obstructed such as a partially closed valve then you need to call a pro. Sorry but we can't give DIY advice here. Delayed ignition is a serious safety matter. The burner has 4 seconds to fully ignite per ANSI stds. Propane is even more tricky. Don't take chances. Shut it down until a pro can service it.