You just need to have that sheet open in CAD, and the Refresh button should read in the changes.
It's possible the system is encountering some sort of error, check the command window for that.
You could also placing some other unplaced detail onto the same sheet, to confirm that it gets correctly marked as being placed. And then delete that one, and see if it gets updated to unplaced.
If it is just that one detail that consistently won't get updated to unplaced, you could use Quick Select to see if there are any other instances of the detail in the drawing. And even just zooming extents in paperspace - it's possible the placed detail was copied off to the side.