A siphon-fed airbrush uses air pressure to draw paint from a jar, similar to how you would pull liquid up through a straw. These airbrushes are often used to cover large areas quickly, and by hobbyists who are experimenting with an inexpensive airbrush. If your airbrush refuses to draw the paint from the jar, there are several things that you can do to solve the problem.
Thin the Paint
If you are working with paint that is not designed for airbrushing, it may be too thick to be drawn up the siphon tube and into the air spray. You can solve this problem by thinning the paint. It is important to use the proper thinner for the type of paint you are using. Water-based paints should be thinned with distilled water, while alcohol-based paints should be thinned with rubbing alcohol. If your paint is oil- or silicone-based, use a thinner with a similar basis. Adding the wrong thinner can cause some paints to curdle.
Inspect the Airbrush
Remove the paint jar and pass a pipe cleaner through the siphon tube. This will dislodge any dried paint that may be blocking the fresh paint from passing through. You can also remove the tip from your airbrush and pass a pipe cleaner through the paint intake and out the front of the airbrush, serving the same purpose. Reassemble the airbrush and spray air without connecting the paint to make sure that the problem isn't somewhere other than the paint.
Increase the Air Pressure
With the paint jar reconnected, gradually increase the air pressure on your compressor by 5 psi at a time. Keep the airbrush's trigger depressed as you do this, pointing the airbrush at a piece of scrap paper. As the pressure increases, you should begin to see paint spraying. Some paints simply require greater pressure to get them moving than others. Stop increasing the pressure when you see a lot of particles bouncing off the paper, creating a haze in the air.
Some paints are simply not designed for airbrushing. You can thin them and increase the air pressure all you want, but they will stubbornly refuse to spray. Disassemble and clean your airbrush to remove any residual paint, and then try a different brand. There are many brands of paint that are designed specifically for airbrushing, with intense pigments to compensate for the fact that they are water-thin.