I had this happen to me one time while I was trying to fry a turkey. Oil was taking too long to heat up but I figured out why. It turns out that I turned the needle valve for the burner on before I opened the valve on the gas tank. For some reason, the burner will not generate much BTU when that happens. So I turned both valves off, turned the valve at the tank on first then the needle valve for the burner, I got a flame like a jet engine!