Thought I had the answer for you, until you mentioned that it had worked previously.
We took our maiden voyage in our new FW last weekend.
Wife couldn't get the refrigerator to run on propane while we traveled.
So she had suffer through leaving the refrigerator off for the hour and 15 minutes it took us to get to our campground.
Then couldn't get the stove to light.
Worked with it for a while and finally were able to get the stove to work with the alternate propane tank.
Never could get it to work on the main tank.
Finally took the main tank out of its compartment and....it weighed about 5#.
The sorry dealer we bought the trailer from didn't fill both tanks for us.