Don
I'm not sure of the exact cause but here are some things to check. Remove manifold and then the offending burner. Check the holes in the burner they should be rectangular, clean the entire burner with brass brush. Then make sure the end of the burner which connects to the manifold is clean. If you have the old style spider guards be sure they are clean. Then move on to the manifold remove and clean each of the three jets (they should not be too hard to clean but be careful not to enlarge the hole) With that done remove the propane hose from the manifold and blow compressed air through the manifold opening each valve high, med and low being sure air comes through at each setting. Once this is done replace the jets and propane hose . Put it all back together and check it. There will be yellowish flames while the burners blow out the stuff you loosened when cleaning it. Then check it again for full lighting. If it works you are home free!! if not then it could be the regulator. The regulator is a standard small regulator and can be purchased at any propane or rv store. Or some times at HD(the HD version will probably cost more than at a propane or rv store).
forgot to tell you after putting it all back together check for propane leaks at all connections with a soap solution(I get a ready made solution at the propane store).
