It sounds like the potatoes are overcooking. . .
Surest way to turn them gluey!
Watch those potatoes and cook then just till they are barely done. Without electricity mash your mixer beaters up and down through the spuds to break them up; the potatoes should look dry and somewhat fluffy, kinda flakey-like. Then put your butter in (or whatever fat you prefer), beat to mix in and then add hot milk/cream and whip. This has worked for me for quite some time now and the times I have ever gotten gluey potatoes are when I don't watch carefully for done-ness and overcook them. This works well with generic "all purpose" potatoes.
Of course your potatoes may be different than the ones we have out here. I personally prefer Idaho grown Russet Burbanks, and if I can't get them, Russet Burbanks from some where else, and if not that, Norkotas work well. It has been kinda nice the last couple of years as the bags of potatoes have been labeled varieties.