If it's blowing air back into the bowl then you've got a clog somewhere. When cleaning an airbrush between colors, or at the end of a painting session, it's common practice to backflush by holding your finger over the end of the nozzle so it will blow air back toward the bowl to force gunk out of the nozzle. This sounds like the problem your having except your not having to hold your finger over the nozzle to force the air back into the airbrush. Try soaking the front of the brush (submerse everything from the bowl or suction tube forward) in lacquer thinner or acetone overnight. Pull the needle out before soaking so the thinner or acetone can get into the nozzle. This should loosen any gummed up paint. Then follow normal cleaning procedures. If that doesn't work get some Airbrush Restorer from Createx and give it another overnight soaking in the restorer. If it still won't clean up you will probably have to buy a new nozzle.
Ben