Turning a canister on its side can help purge air... It's a valid suggestion, although I don't know how effective it would be with an FX5 as I have never tried it. However, that is literally the ONLY way I can get my XP3 primed.
It is sucking air. take the lid bad off. then screw it back down by screwing the next screw across from the last one screwed until all are snug (be careful not to over tighten).