I wouldn't use Windex to thin with, it cleans up Tamiya real nice!
I've never had any problems, and Tamiya and Future is all I use. I think your flat coats might be going on too dry, which causes them to be TOO flat. You can tell if this is happening by feeling the surface, if paint dust comes off on your fingers, or it feel almost like sandpaper then it's drying before it adheres to the model and the Future has to be built up in multiple coats, and it STILL might not overcome the problem.
I spray the Future one is a sopping wet coating and they come out shiny like a new dime!