If your mashed potatoes are chunky or difficult to mash, chances are they are undercooked. This can happen if you chopped your potatoes into chunks larger than 2-inches or if you used the stovetop method and drained them before the potatoes were fork tender. To fix this, cook the potatoes over low heat or using the sauté setting on the Instant Pot for a few minutes longer until fork tender. Then, mash with the remaining ingredients per usual.