I am Canadian and I usually get potatoes from PEI. My girlfriend who is American brought me a bag of Idaho white potatoes and I mashed them. I washed and peeled them and cut them in half and boiled them for what took about 50 minutes! In the last 5 minutes, they went mash all right. They absorbed all the water and turned out water mash potatoes, I barely had to mash them! Is it that Idaho potatoes don't mash well or because they were white potatoes?
I have experienced this too with one variety of russet potatoes. It just won't finish cooking or has hard spots in the potatoes that don't seem to break down. I really have not been able to find out what causes it. I have never experienced this with Idaho Russet Burbanks, which I personally always look for when baking or mashing. The bag (probably long gone by now) should have the variety either printed on the bag itself or a quick lock mechanism that attaches to close the bag. This variety labeling is only done by Idaho. The varieties being sold into the fresh market currently are the Classic, Russet Norkotah, Russet Burbank, Umatilla and Alturas.
I also have the best experience with russets by peeling the potatoes, cutting in half lengthwise and then into smaller 1-2 inch chunks then starting from cold water and boiling. This evens out the cooking. Often a large russet will be done on the outside way before the center if soft enough to use for potato salads or mashing.
Click here to see the varieties Idaho sells.