This is my decal sequence
1) glosscoat, a smooth surface prevents silvering
2) put decals on, I use vinegar/sol/set to soften the decals
3) with really stuborn decals, use future a setting solution
Then finish the model (gloss coat, wash flat coat) as wanted.
You are not the only one having problems with decals, for more idea's look in the "decal"section of the forum.