How can I stop duvet covers “balling up” in the tumble drier?
You can’t really. Some tumble driers come with a reverse action that helps to a certain extent, but big items almost always do this. Make sure you give the bedding a double fast spin in your washing machine to get them as dry as possible, then hang the duvet cover over a door or upstairs bannister rail for a few hours, once most of the damp is out, finish it off in your drier to take out the creases and fluff the fibres nicely. If you have access to a washing line outside, use this to get the damp out, but not everyone has this luxury.