Are you in a subfolder or collection? I think DnD works most places, but not in All Photographs, Higher level folders/collections or Previous Import. It works in subfolders, subcollections, quick collection and previous import.
The reason you need to be in the lowest level is you could be moving things from different folders/collections. If you need a lot of things from different folders/collection moved around, create a new subcollection for these and sort them there.