Alfred,
I've had the same experience with decals I've printed recently (and also for the first time) with the Testors Decal System. I made woodgrain decals for the cockpit floor of a 1/74 scale biplane twin-engined bomber I'm working on. With the open cockpits, the floors are easily visible and I wanted them to look really good.
I realized pretty quickly that I needed some sort of light-colored background when I put my first decal down directly on the dark plastic of the kit. I quickly took the decal back off and resigned myself to the fact that there are no shortcuts with this kit.
I primed the pieces with enamel, then put down Testors MM Acryl radome tan. After that dried, I coated the pieces with Future. Having done all of that, the decals went down beautifully and responded well to Micro Set.
In my opinion, these decals need a light-colored background. One tip I read on a similar thread on this very topic is to print the decals onto clear plastic used for overheads and use them as masks. The idea is to cut out the decal leaving a hole in the plastic with the same outline. Then paint a light color background on the model in the shape of the decal. After that, just apply the decal on the background.
Regards,