There is a required sequence for turning valves on to prevent gas flow if something is disconnected. Not following that sequence might be the cause of your problem.
Make certain that both the tank valve and the regulator valve are off:
Open the tank valve completely.
Open the regulator valve and light the burner.
Close the tank valve.
Close the regulator valve.
If you have the regulator valve open when opening the tank valve the safety shutoff will stop or limit propane flow.
Here's a link to a site that explains it more fully: http://www.classicwinnebagos.com/for...p?topic=6999.0