Are you coloring a lot of tiny spaces where you are holding the marker totally vertical only using the very tip of the marker?
I had a situation where I was coloring a lot of tiny spaces and my dark blue started looking shiny and gummy. Colleen told me to carefully remove the nib and soak it in alcohol until it turned very light. Then put the nib back in the marker . When you put the nib back in lay the marker on the table and remove both end caps from the marker so the ink can equalize. It worked like a charm.
She said the dark colors have a tendency to get gummy if you use just the tip of the marker. You also need to use the side of the nib to keep the ink flowing.
Now I have a piece of scratch paper and periodically stroke the side of the tip on it when I am coloring a lot of small spaces. It keeps the ink from getting gummy.