There's 2 issue:
1.the setting solution just makes it conform to the plastic, you need to get the 'shine' of the decal to match the rest of your paint. For example, I do airplanes, so after the flat paint goes on, I spray gloss (or future) coat, put the decals on, after the thing drys, spray flat again to make everything match.
2. Some decals are thick, in those cases even after the shine matches, you can still see the outline. For those, you just have to trim away the clear parts before you apply the decal.