hkshooter:
My limited, non-chemistry knowledged, trial and error based experience is that acrylic (Tamiya) don't like to stick to anything as every time I mask the stuff it lifts. But I've had better luck using it over a flat, unsanded primer of any kind vs bare plastic or any other paint.
I've never had significant problems with Tamiya acrylic not sticking, and I don't often use a primer. I have to ask, what are you thinning your paint with?