Base coat problem

You are using Tamiya enamels? I didn't think Tamiya made any enamel other than their sprays, but I'll take your word for it. I've run into that problem using some colors of Tamiya acrylics (specifically black), but never with any kind of enamel.

I'm afraid I'm not going to be much help, but the cause must be that your dry brush paint is softening the base paint for some reason. Are you using a different thinner or brand of paint? Anything different other than painting with a single thin coat? If not I'd guess that the single coat is part of the problem, although I don't understand why. If the coverage is good and the paint is cured I don't see why that would cause any problems.

Wish I could be more help, but that's really all I can think of.

Yeah i think that i should do 2 coats ' cause the white primer was still showing on the raised areas. The humbrol and tamiya enamels seem to mix together fine. Tamiya enamels are in the square bottles here in NZ i buy them purely because the bottles are a better design than the humbrol Tins which get gummed up with paint to easily( i know i should clean them properly).