Most of the rasp-style graters we tested come with a cover for the grating surface—and for good reason. The graters’ teeth actually poke outward slightly—that’s how they bite into the food. These 3-D teeth can flatten out over time as the graters get thrown into drawers or slid into utensil holders. If the teeth on your Microplane Zester/Grater feel dull, it’s probably not that the teeth’s blades have dulled but that the teeth have flattened, so they don’t have the same bite.
To fix this problem, you can go hole by hole and poke them back out if you’re feeling especially coordinated and have lots of free time, but we prefer the preventive route.