Are you using th Model Color or th Model Air ? I actually thin both, but th Model Color requires a little more thinner. I use th Black Capped bottle & not Their Airbrush Thinner. I mix it maybe 60/40 - Thinner to paint for th Model Color & 30/70 thinner to paint for th Model Air & get a good flow. ..... It'll still want to collect on th tip of th AB, but that can be remedied with th use of a #0 , or #1 brush dipped in water & swab around th tip , blow off th excess & then you can carry on.
Also .... If you are tryin to shoot a Tri Tone - be it a German or a Nato/Modern , & need to get a tight pattern , you'll want to thin it a little more & back off of th pressure. I shoot with a Chandler & Thayer Omni 4000 @ about 20-25 psi , & to get small tight patterns , drop to 15psi. & thin it almost 80/20 - That's what works for Me .....
I've also heard & it seems to Me that Vallejo operates a lot better in a Gravity Feed AB. WIth a Sifon feed it tends to get a "skin" on top , whether in a cup or bottle. - My personal experience.
There is a bit of a learning curve, but once you get it ... You got it !!
WHAT DO YOU CALL THE ASSASSINS THAT ACCUSE THE ASSASSIN???