The air valve won't open ?

That's usually from paint getting atop it and basically gluing it shut  , but it sounds like you have inspected it so I imagine you would have seen any paint on it .

I don't know exactly how that Iwata is constructed but I doubt the O-ring can move once the valve assembly is screwed into the AB body .

This is the airbrush ? :

[ATTACH=full]94301[/ATTACH]