for many years I have made mashed potatoes without any problem and never have they been lumpy mashed potato. I buy just brushed potatoes from the supermarket, cube them and put them in a large bowl covered with water and cook in the microwave for 20 minutes.
About a month ago, I cooked the potatoes as I have always done, and started to mash them and realised that they weren't all cooked and therefore the mashed potatoes were lumpy - first time in my life. (I am 66 yrs of age). About a week later, bought from same supermarket, cooked the same, again not cooked completely again lumpy.
Then last week, my grand daughter (24 yrs) cooked mashed potato and she had the same problem. I then tipped some of them into a smaller bowl and cooked them for another five minutes. Still not cooked. I then heated up some oil and put some of the cubes in the oil and cooked them. While they browned on the outside, they were still not cooked through.
Outside United States food preparation
Let's start with the right variety of potatoes for mashed potatoes.Some potatoes are better than others for boiling, depending on what you plan to do with the cooked potatoes. This is because the starch content varies, which affects the texture.
- High-starch potatoes, such as russets, have a light, mealy texture. Once boiled, they are ideal for mashing.
- Medium-starch potatoes, such as Finnish yellow and Yukon gold, contain more moisture so they don't fall apart quite as easily as high-starch potatoes. They work in the "okay" category for both mashed potatoes and potato salad
- Low-starch potatoes, such as round red, round white, and new potatoes, are sometimes called waxy potatoes. They hold their shape better than other potatoes when boiled, making them ideal for potato salads.
Next, let's look at the method. Potatoes in the microwave do better when they are microwaved whole and then peeled and in your case, mashed afterwards. Potatoes are high in starch and foods high in starch - rice is another high starch food - microwave better when they can be stirred to evenly distribute heat. As we can't do that with potatoes and even cubes of potatoes can cook unevenly, uneven cooking can happen.
Short answer - check the variety and try another method of cooking.