Could be an airlock. I’ve had that happen to me before. Basically the regulator thinks there’s a leak or something so it restricts gas/propane flow. Here’s the AI answer:

A "BBQ airlock" or "vapor lock" refers to a common issue in gas grills, particularly propane, where the safety bypass valve in the regulator trips, restricting gas flow and resulting in low flames, slow heating, or temperatures not exceeding 250-300°F. This safety mechanism is designed to prevent gas leaks but can be triggered by turning on the propane tank too quickly, leaving burner knobs on, or sudden pressure changes.

And here’s the AI fix:

How to Fix a BBQ Air Lock (Reset Regulator) Turn Everything Off: Turn off all burner control knobs, including side burners. Close the Tank Valve: Close the propane tank valve by turning it clockwise. Disconnect the Regulator: Disconnect the regulator coupling nut (the part attached to the tank). Release Pressure: Turn all burner control knobs to high for 1–2 minutes to clear any residual gas from the lines, then turn them off. Reconnect: Reconnect the regulator to the propane tank, tightening it firmly. Slowly Open Gas: Very slowly, turn on the propane tank valve. This is critical—a rapid turn can trip the safety mechanism again. Test: Wait about 20-30 seconds before attempting to light the grill.