Sounds like pigment separation. This can happen if you dilute the paint too much, or if you let it sit in the cup too long, sometimes even while spraying. This is a particular problem with colors containing white pigments like grays.
Tamiya acrylics should be thinned ~40 to 50% with either Tamiya thinner or 90+% isopropyl alcohol—about the consistency of 2% milk. Don't thin in the cup. Thin in a separate container.