I've had similar problems. Gloss it up for decals, and I can never get it really dead flat again, sometimes. Sometimes I can. Can't figure out the variables. Weather? Barometric pressure?
Quite frustrating at times.
The only thing I've used that busts it dead flat every single time is Gunze Sangyo's lacquer-based flat. Only problem is, you can't use it over acrylics or decals!!!! (Or so they recommend on the can. I've used it on both, and you get kind of a cool dusty look. And dead flat!)
Some armor does have a bit of shine to it, though...sigh.