Grill does not heat over 250 degrees

Here is a couple of things to do.

Make sure your tank is filled.
Straighten Fuel hose, so it is not bent or kinked.
Does your front burner light with a match? If it does light then check the Crossover Ignition System.

Also, It could be the way the unit is turn off.

Try following thses steps, this will help to reset the excess flow safety device.
Turn all burner control knobs and the cylinder valve OFF.
Disconnect the regulator from the cylinder. Turn burner control knobs to HIGH. Wait at least 1 minute. Turn burner control knobs OFF. Reconnect the regulator to the cylinder. turn cylinder valve on slowly. Then go ahead an light your grill.

Fireplace-girl

very likely if its an older bbq, the burners are full of rust and need to be replaced. nextthing the pressure regulator (on the hose that goes on the tank) is faulty) last thing to check would be the jets (under control dials)