Duvet bunching up

[Twilightstarbright]

DS’ single sized duvet keeps bunching up inside the cover and I don’t know why- I don’t have this problem with any other duvets. Any ideas?

Does she have a polycotton duvet cover. I have this problem if I don't use plain cotton covers.

I think it's the wriggling! Happens to dc2(10) & dc5(4) duvets too, no issues with our bed other the other 3 children. It drives me mad.