Your best bet would be to first strip down your airbrush and give it a thorough cleaning. It sounds as if it's pretty well clogged.
I tried thinning the paint with Vallejo AND Tamiya acrylic thinners, but the results are the same as before; tiny little pigments just floating on the clear liquid..
No, don't do this. Tamiya thinner + Vallejo paint = sticky ball of goo.