Future problem

I find that I get better results by spraying a another gloss coat after applying the decals.  By doing that the surface of the decals will become the same as the rest of the model and will be dulled in the same manner as the rest of the model.

As far as the flat coat, I've had inconsisent results with DullCoat.  Some batches worked really well, but others gave only a satin or semi-gloss.  I've gotten much better results by mixing Tamiya X-21 Flat Base with Future at a ratio of 4 parts Future to 1 part Flat Base.

The best solution is to apply Future to the whole surface (say a wing, or at least the panels under the decal) then apply the flat. Sometimes that doesn't work either because of differences in refractive indices of the various clear coats. 

Was your dullcoat thoroughly mixed? The most frequent cause of a gloss/semi-gloss finish when using a flat coat is insufficient mixing.

If using a spray can this means a minimum of 3 minutes shaking, preferably more. If airbrushing, it means thoroughly stirring in the jar before decanting into your mixing jar, then adding thinner and stirring again.

A common problem when using a "spot" application of Future (or any clear gloss coat for that matter) is that it can appear to darken the base on which it's applied. While it diesn't actually change the underlying colour it alters the reflectivity of the surface and makes it appear darker. Sometimes when this happens, applying a flat coat over the top won't bring it back to the same apparent shade as the surrounding area.