I think there is an overheat relay that will shut it down if the shroud gets to hot. Make sure you filters are clean the combustor head isn"t restricted with crud. A lot of these are used in costriuction sites where theres saw dust in the air and they can get pretty clogged.
I bought one off a guy last monthh and it would light. That turned out to be due to both a cracked fuel hose and a cracked air line. The filters were clogged with saw dust and their was fine dust baked on a lot of the components. I took it all apart, cleaned it up, replaced the hoses and some other misc parts and it runs like a champ.