shiny spots in decals

When applying a decal, it should be going over a glossy surface, most like to put future down, then decal and etc...

I usually spray my model with future, put the decals washes etc, but before I seal it up with a flat coat, I use micro sol and set when I apply the decals. This helps the decal conform to the model surface.

 

the shinny spots in the decal is because air bubbles have been trapped in between the decal and model surface.

 

As far as I know, once the decals are sealed - like you said you have done, there is no way to fix this. 

OK, this is just a guess, but it could be that the matt coat is being sprayed over a decal that has residue of different mediums in different locations on it and your matt coat is incompatable with one or more of those. After I apply decals, I wipe them (actually it's more of a "rolling pin" motion) slightly with isopropyl alcohol on a cotton swab (cleans them & helps them comform), then spray on a coat of future before my flat coat is applied.