Peeling paint--what am I doing wrong?

First of all, drop your airbrush pressure. For siphon fed, about 15 to 20 psi or less. For gravity fed, 10 to 15 psi or less.

Thin to the consistency of 2% milk—a little thinner than you probably have it. 

For washing, wash the parts (assembled or not) just before painting with Simple Green or a surfactant cleaner (Windex works too). Rinse first with tap water, then rinse with distilled or deionized water, followed by a wipe with either 90% isopropyl, ethyl, or denatured alcohol.

The reason to wash just before painting is that the plastic contains compounds called plasticizers that slowly migrate to the surface. Between the time you wash the parts on the sprues and the time you paint, more plasticizer may have migrated to the surface. Some plastics contain more of these compounds than others. Different manufacturer's plastics may perform differently, or even the same manufacturer's plastic may (does) vary over time.

Also, some manufacturers use more mold release than others. Depending on the composiiton of the styrene, the temperature, and the dwell time in the mold, there is a potential for the plastic to absorb mold release agent near the surface of the mold. This will also slowly migrate to the surface over time. 

You should wait a minimum of 24 hours between coats of acrylic paint, especially if you are masking over a coat. Acrylics dry quickly but cure slowly. They do not develop full bond to the substrate until they are fully cured. Do not mask until the underlying paint is fully cured.