I think zokissima may have hit it there with the compressor contamination.
I'm also thinking, though, that the 70% iso alcohol may have some other ingredients that have left a residue on the plastic. The effects you describe sound very much like the surface being tainted with a silcone or waxy substance. Does the Iso you're using contain any kind of menthol? Don't know for sure but that may be it.
When I spray onto bare plastic I only use 100% denatured alcohol. This won't work, however, on painted surfaces. Try taking an old model or a small piece of sheet styrene and clean it in the same way as your model, with the exception of treating only one half of it with the iso acohol that you are using. Spray the whole piece, and check the results.
Let us know what you find out.
stinger