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.