tcepilot you many still have a bit of smuch (an air brush tech. term 
) in the passageways.
Try to take it apart and give it a deep cleaning with lacquer thinner and small brushs. I carefully use dental brushs (@ $ store) and gently swab out the openings. don't forget the airways as well as the paint ways -the cup and the air brush's.
This isn't brain surgery but you have to treat it like a delicate operation- no hammers allowed 
.
As you reassemble keep aware of how parts go together, they should fit easily and come together easily- if you have to push or twist it hard, then something is wrong- back off and carefully try again.
I'm sure you know all of this but sometimes you fool yourself into 
the brush is clean and it isn't (dont ask how I know 
).
Then as the others stated check you painting technique - tipping the brush etc.
Good Luck and keep up your good work - your Vietnam Air War GB is looking fine .