After soldering, inspect the solder seams for small areas or strips of copper foil edges that aren't covered with solder.
If this exposed foil is where you want the solder bead to be, you need to clean the foil and re-apply solder. Usually scrubbing with “000” steel wool is sufficient. If, after scrubbing and applying flux, the solder still does not stick, you need to wash the piece and after drying, scrub the exposed foil, re-apply flux and solder again.
If the exposed foil is surplus or where you do not want any solder, take a craft knife, and carefully trim off the exposed foil.