There may be a safety valve in the tanks gas valve assembly. If the velocity of the gas through the valve is too high (example-the gas hose gets cut) the valve may stop or limit the flow of gas. Close the valve, relieve the pressure in the hose and then re-open the valve VERY SLOWLY, just enough to light the burner, then turn it up. I had this problem with a 500K BTU wand burner. Good luck.