This recipe works for me every single time I make it. I often cheat by peeling and boiling the potatoes and allowing then to steam dry in a sieve and they still work every time. The trick is to make sure they are dry before mashing them which is why steaming is preferred.
Steam the potato until soft and then remove skins when cool enough to handle but still warm, then mash, allow to cool. Once the potato is cool stir in the flour, add the egg and salt and kneed or a soft dough.