You may not have actually gotten all the adhesive off. There is often a ridge of hardened, dried adhesive at the edge of the decal which is very difficult to remove. There can even be dirt that got embedded at the edge of the decal that combines with the adhesive to make a sort of concrete. To remove it you have to use your chemical, usually a citrus solvent and try to keep it wet with the glue solvent for a while, like 15 minutes (using a paper towel wetted with the solvent held over the area) to soften the ridge, and then use a plastic tool like a vinyl squeegee or a plastic razor blade (be very careful with the razor blade) to scrape away the ridge of hardened adhesive. This can also be necessary if a decal has been removed but the adhesive not removed immediately.