1.Sounds like the paint's too thick. Try thinning it.
2. At what distance from your model are you spraying? May want to get closer.
3. You may also want to try straining your paint to get thickened pigments out of the way. An old pair of pantyhose works really well for this.
4. Ensure your paint is well mixed prior to thinning and adding to your airbrush.
5. Make sure the siphon tube from the paint reservoir to your airbrush is thoroughly clean and open.....Likewise the airbrush itself.
6. Make sure your paint is a fairly new batch and hasn't been sitting on your shelf since the Civil War.
These are just some thoughts and ideas. Try what you like and see what works.
Gip Winecoff