I never found K&B to be a very effective solvent in general. I hated using it even for RU. Mine dissolves completely in Kirkland minoxidil at least to my eyes. If you're talking about tiny almost imperceptible drops of transparent residue on the surface, that might be solvent impurities. If it's dusty to a significant extent, try dissolving some in some Kirkland minoxidil or 50 PG / 30 Eth / 20 water to compare.