It really could be a number of things. the good news is Glow engine are so simple - they need air, fuel and heat to run. To much or not enough of either and you will have problems. So, checked for blockages, checked for leaks, checked the plug..... how about a pin hole leak in the tubing being uncovered when the nose is up? But it could also be the o-ring. You are probably going to have to eliminate things one by one