It MIGHT be the thinner. Which airbrush are you using specifically, and which tip are you using it with? Are the acrylics the ones with the white labels (Acryl)? Rubbing alcohol or isopropyl alcohol?
Acrylics generally require a more open tip: For the Azteks use a white tip at least. I can spray the Acryl unthinned with it at about 15-20 PSI, I use an A300 compressor. The flats and semi-gloss ones go down beautifully, but I'm still fiddling with gloss paints, might start thinning them depending on how I can get them to lay by varying my technique.