here is something i found for you
Problem: I have a suction type airbrush and am not getting paint out of the nozzle…I use the bottles that came with the brush…I am getting air, just no paint. Solution: Check to make sure the vent hole in the lid is open, also make sure the pickup tube is not clogged or sitting flat on the bottom of the jar. Remove the lid and attach it back to the airbrush, stick the suction tube in a container of water and see if it sprays water. If it does then either the vent hole is plugged or the tube is sitting on the bottom of the jar. If it wont spray water, check the tube for a blockage, or the inlet for the tube into the bottom of the airbrush. Once these are clean you should be able to spray with ease. Also make sure your needle is sliding back and forth and not slipping in the needle lock nut.
Problem: No paint is coming out of my airbrush, I have gravity and bottom feed airbrush gun and I also notice bubbles in my bottle and cup. Solution: Check Fluid nozzle, needle or nozzle cap. When any of these are clog you will get back feed back into cup or bottle.
found these here http://howtoairbrush.com/airbrush-problems-and-solutions/