Went through this with one of our workers yesterday as a matter of fact. She changed the bottle, but still the fork wouldn't start. One look at the valve told me the connection wasn't on properly, therefore no matter how far you turn the tap on,, it won't work. She'd opened it right up, although it was easy to undo again.
I explained to her the possibilities, and my experience the other day of fitting a bottle which had the tap already open.
Decsission time,,, do you go for the tap, or try and undo the connection again. Of course, connections always get jammed when you least want them to [bigsad] Gas burns hurt like hell.

