Metallic paints have pigments that are quite large. In many cases this pigment is just too large to pass through some airbrush nozzles. This problem is made worse by airbrushes with a very fine nozzle. In some cases you can remedy the problem by reducing the pressure. Reducing the air pressure allows you to move the needle back further yet maintaining the same volume of paint flow. When the needle is moved back further more of the nozzle is open, thus allowing paints with larger pigments to pass through.