Gluey or gummy potatoes are the result of over-mashing the potatoes. When you use a traditional masher or a potato ricer, the starch molecules stay intact. But when you use a mixer, it tends to tear the starch molecules, releasing the starch into the mixture. The starch will absorb liquid from the mixture and become "gluey" or "gummy." If this happens, it's a little too late to fix--but throw a lot of cheese in there and call it cheesy mashed potatoes for a delicious side dish.