Why does drag and drop often not work?

[Jeannie Bee]

Word 2007 - sometimes I can drag and drop to move text but other times it
doesn't work. I highlight the words, get the white left-pointing arrow, keep
mouse button depressed but there is no dotty line to drag to the new
position. Closing Word and reopening doesn't solve the problem.

 

[Peter T. Daniels]

When certain dialog boxes, such as Insert Cross Reference, are open,
drag'n'drop doesn't work. (You get a slashed circle, like a No Parking
sign, instead.)