In reply to Gazlynn:
I’ve had this problem – it wasn’t a Jetboil but if you have the same problem as me it can apply to most gas stoves. I had a tiny bit of grit or dirt in the pipe that delivers the gas from the cylinder to the burner. I took it apart as much as I could, open the tap fully and stuck a tiny bit of wire through to clear it – I could feel the blockage at first then felt it clear (blowing didn't work for me).
Getting the wire was the hardest part – I was in Italy at the time and don’t speak the language at all. I went to a hardware store and after about 20 minutes of sign language with sound effects got about two inches of electrical cable and pulled the individual strands out to poke through.
Having said all that I just had a very quick look at my Jetboil and it doesn’t look as easy to poke something through – but I’ve written this now so I might as well post it - it might help.