I think a good cleaning will correct your problem. Looking at the provided excellant pics by Joe and old-biscuit break the connection at the flare nut and orfice nut. Remove the single Philips head screw holding the burner ass'y in place. You will have to wiggle the burner ass'y to free it from the end-slot, see the pic Joe provided, the flat end can be difficult to remove the first time. Remove the orfice nut from the burner tube.
Clean the orfice w/alcohol and blow out. A slight touch of anti-seize on the orfice nut threads will make removable easier the next time. Clean the burner slots and inspect the inside of the burner for mud dauber nest. Reassemble all and light it off.
Other things to consider are improper propane pressure and a stopped up flue.
