Shock shafts, just by their nature attract dirt. As the shaft goes in and out it has a little residue from thr resevoir. This residue is slightly damp and attracts dust and dirt. The easiest way to clean it off is with an old toothbrush. Also if the small amount that it takes to attract dust is also diminishing the amount of shock fluid in the resevoir. I clean and refill my shocks after a few times of running and always just before the main.