Technically, it's the size of the pigment in the metallics that causes the problem. The needle of an airbrush fits into the nozzle and effectively seals it when the needle is completely closed. When you pull it back to allow paint to flow you are barely opening a crack between the needle and the nozzle. The particles in some metallics (as well as some other large-pigment paints) are just too large to fit through the crack. Thinning won't help because it's the actual pigment particles themselves that are too large.
Two things you can do: 1) Spray at a lower pressure. This allows you to pull the needle farther back for a given volume of paint flow, thus making the "Crack" a little bit bigger. 2) Use a larger nozzle . needle combination. Some airbrushes only have one head assembly available, others have several to choose from.
I had problems with Alclad until I reduced my pressure to about 8 psi and that made a world of difference. At that pressure my airbrushes are basically wide open and the paint flows very well.