Copic Marker nibs keep getting stained?

I have been using Copic Markers as I am a carttonist/illustrator....Howhever, my nibs on my lighter colored markers (Eggshell, Milky WHite, Maize, etc.) keep getting an ugly black stain on the tipnof the nib. This doesn't affect the actual color of the ink, but it is really annoying. I only use sakura microns and the actual Copic brand multiliner SPs. My nibs, nonetheless, keep getting stained black...