First , always mix your paint in a separate container then add it to the paint cup of the airbrush .
I cut the bottom off of small pill bottles like aspirin bottles etc and use that for mixing .
Vallejo can be very inconsistent in viscosity between colors in their Model Color line of paints so you will need to adjust the thinner/ paint ratio accordingly .
Their AB ready paint is called Model Air .
2 parts thinner to 1 part paint seems really high , but I can see that being necessary with some of their colors but not all .
Make sure you mix the paint in it's bottle thoroughly .
Remove the dropper top and stir it .
These are the two thinners I would recommend for thinning Vallejo Model Color for airbrush :
[ATTACH=full]94299[/ATTACH]
What size spray tip and what air pressure are you running ?