Another possible reason would be the presence of amine blush, a waxy substance that can form on the surface of cured epoxy. If it's present, new epoxy may not bond properly with the cured surface. While scrubbing with hot water and a mild detergent is the usual technique for cleaning, sanding also helps, as it both creates 'tooth' for better mechanical adhesion and exposes blush-free epoxy beneath the cured surface.