Potatoes Au Gratin Update

Just arrived home from dinner with the family where I served my Potatoes Au Gratin dish. For some reason the potatoes remained firm - didn't cook up and soften as they should.  I've made this recipe a lot - in fact, I think the last half dozen times I made this dish I used Yukon Gold. Anyway, for what it's worth, the Russet potatoes I bought at the store today and used in the casserole did not cook up as they should. Everyone ate them anyway because the flavor is so yummy, but it was disappointing just the same.