Hello Claire, I would be happy to help you troubleshoot this issue. The main reason homestyle mashed potatoes turn out sticky and dense is the release of excess starch, which usually happens when the potatoes are overworked or processed with the wrong tool.
When you use a food processor or an immersion blender, the high-speed blades break down the potato cell walls too aggressively, turning the starch into a glue-like paste. To avoid this, it is best to use a traditional hand potato masher, a food mill, or a potato ricer.
Additionally, the choice of potato matters a great deal. Starchy varieties like Russet work best because they yield a naturally fluffy texture. Waxy potatoes contain too much moisture and less starch, which can easily lead to a gummy consistency when mashed.
Finally, make sure not to overmix when adding your warm butter and milk. Gently folding them in just until combined will keep your potatoes light, smooth, and perfect for your family dinner.