Airbrushing paint finish consistency...

Thinning may be one of the variables, and you might want to try thinning a tad more. The other variable that comes to mind is painting distance. Sounds like the pigment is drying before it hits the model. You might want to move your airbrush closer. All this, of course, is going to depend on how much air pressure you're using, and how fine a setting you have on your 'brush. In addition, make sure your paint is thoroughly mixed prior to and after thinning. Depending on how long you spend spraying in one session, you may also want to occasionally stir the paint in your color cup or bottle to ensure a homogeneous mixture throughout.
Hope this helps.
Gip Winecoff