Putting lubricant in the air valve assembly.
Just don’t do it! Nope. Never. Most lubricants will just leave a residue that will make your airbrush sticky in the long run. Dry is good. Residue is bad. If you feel that you HAVE to use a lubricant (and there really is no good reason that you would ever need to), use glycerin, because it leaves no residue.
Additionally, if you start lubricating your air valve regularly, your airbrush will require it to continue working the same way. So once you start using it, you will forever need it. Just say NO!
The next time your air valve becomes a little sticky, first try cleaning it, then allow the air valve assembly to dry thoroughly (to allow the O-ring size to stabilize).