Vallejo paint, all varieties, have in fact only ONE slightly tragic flaw:  The bottle.  While the eye dropper has revolutionized paint dispensing, the fact is the bottle simply doesn't have enough room in it for shaking alone to properly homogenize such a viscous paint.  While ModelAir colors are usually thin enough to negate this, ModelColor and GameColor, being decidedly thick by definition, make it a real chore.

So while you may think you have it shaken up well, and thinned it till it looks good, there is more often than not little thick bits in there, hiding, waiting for to reach the constricted orifice of the airbrush...and what started off as a pleasant AB session turns into frustration.  In desperation you dump it out, clean out the gun, start over only to run into the same thing sooner or later.

I speak from long experience here.

Possible solutions:

1 ) Use ModelAir.  Yea, I know, I too had a zillion ModelColor shades when I figured this out but over time switched my buying to ModelAir as need be and now I rarely reach for ModelColor when airbrushing.   Even then its worth noting you still need to shake those bottles like a mo fo, in alternating and random directions, before dispensing, and when doing particularly delicate work you may still want to add a bit of the clear thinner.

2) Mix/thin in a separate vessel BEFORE dispensing to the air brush.  Use an old paint brush, not a stick, to mix it....and mix and mix to make sure you get it all homogenized.  Then and only then load the gun.

3) For particularly thick colors, take the eyedropper off the bottle (just pulls off), get a tooth pick in there (or other suitable stick), stir and break it up GOOD, then put the dropper and cap on and shake till you arm is soar...then go to suggestion #2.