Cannot move items in Item Sets (MAC, OS-X)

So I'm experiencing some sort of bug that has been annoying me for 3 years now. It's not always that I can't move the items. It usually fixes itself after a couple of weeks. But it's really annoying to have to wait sometimes. I don't know what causes this and I don't see anyone else that's having the same problem as I do at the moment on mac. But if anyone knows let me know, would really appreciate it. :-)