Decals are designed to adhere to a smooth surface. Gloss paint is smooth. The reason it looks shiny is that the light is reflecting off of that smooth surface directly to your eyes. Flat paint and in this case semi-gloss paint is not smooth, the reason it looks flat is because the light is not reflecting directly back at you but is being reflecting in all directions off of the bumpy surface. So, this a long way around to explaining why your decals are silvering. They won't adhere to a flat 'bumpy' surface because as others have stated, air is getting in between the decal and the paint.
The solution? Put on shiny paint, put on the decals and then hit them with another coat of gloss clear to firmly adhere them (this coat may be optional?) and THEN put on your semi gloss or flat paint. Problem solved. I hope this helps. -RRR