You can apply Future over any fully cured coating you are likely to use on a plastic model. Fully cured, for the thick coating you seem to have applied, may be weeks for acrylics, at least a week for enamels.
But it sounds like you are airbrushing too far away and/or at too high a pressure. This causes the paint to start to dry before it reaches the surface. The other possibility is that the paint is too thin or has stood long enough that the pigment has settled somewhat. The increased amount of pigment in the applied paint will reduce the gloss.