One thing that always works is to carefully trim away the clear decal film, but this can sometimes be wery tricky.
Hopefully someone else with more experience with decals will swing by with a solution
the steps I use when decaling (step 9-10 and 12-13 = extreme cases) are :
1 glosscoat the model (enamel)
2 dip the decal in warm water
3 apply to model
4 when in the right place remove excess water with a bit of paper and press down the decal as much as possible.
5 apply micro sol
6 wait
7 repeat 5 and 6
8 if still silvering poke holes in the decal using a needle and apply micro sol
9 still silvering? poke more holes and use a brush to squeeze ''gloss''-paint into the silvering area.
10 let dry
11 overcoat with gloss (enamel)
12 wtf?? still silvering... apply some kind of alcohole (softens the paint) poke holes in the decal and try to get more gloss into the silvering area.
13 use fine grit sandpaper to even out the finish
14 apply matt or semi matt
15 done!!!!