Your symptoms match what I once experienced with my Future/Tamyia Flat Base mixtures when I had the ratio of Future to Flat Base to low with too little Future and too much flattening agent. In the areas where the flat mixture would pool, i.e. corners and crevices, the flat base would come through as a snowy coating.
If this is the problem that you are experiencing, it could be that you need to shake up the rattle can more prior to spraying into your color cup. Since the flatteners will settle and the pickup tube in the rattle can draws from the bottom, you may well be getting flat mix that has too much flattener.
Alternatively, you might consider using a regular flat finish for your airbrush. I use a mixture of Future and Tamiya X-21 Flat Base mixed in a 4:1 ratio of 4 parts Future to 1 part flat base. Another highly recommended product is PollyScale's flat acrylic. I've never used it myself, since my local hobby shop doesn't carry it, but I don't recall ever having read a criticism against it. Testor's Dullcoat is decent, although I've had some sample to sample variation with some samples having a sheen to them rather than being truly flat.
Andy