First, welcome to the boards! There's nary a question out there that someone here can't answer.
Now, on to your specific problem. It could be a few things, so I'll throw out what comes to mind.
1) Tamiya acrylics don't like to be brushed on - especially multiple coats. Unless you give the first coat plenty of time to dry (48hrs), the second coat will tend to roll up the first when applied. Even if you're overlapping, the portion that's overlapping will roll up. Tamiya acrlyics (when not properly thinned) skin over fairly quickly. And what's happening is the brush is pulling up the paint that has skinned over but not dried. It's a real pain in the butt. Try thinning the paint in a small cup first about 2:1 (paint:thinner) or dipping you brush in thinner, drying some of it off and then in the paint.
2) If this is happening on bare plastics, there could be mold release agents or oils from the plastic itself on the surface. Did you spray down any primer or wash the model in warm soapy water first?
I'm sure you'll get better answers from more knowledgeable people, but those should be somethings to start looking at.
-Fred