There are a few common reasons.
Spider nest at the burner orifice. Stink bugs in the burner tube. Air in the gas line. Bad thermocouple.
In this order and attempt to light as you go. Crack the gas line at the gas valve till you smell gas. Remove the burner tube and check for stink bugs or spider nest. Lastly attempt to light while the burner shield is off so you can see the flame. If it lights for a second or 2 and goes out make sure the thermocouple is in the flame. If it is and still will not stay on replace the thermocouple.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles