Can't delete text box in PowerPoint

I've recently had a problem deleting text boxes in PowerPoint.  I'm able to select the box by clicking on the box perimeter (which changers the little circles and squares at the perimeter from open to solid, as it should).  But when I hit delete, it deletes the last text character within the box, rather than the box itself.  I also can't copy the box.   When I select the box and hit command-C, I hear a little whish, signifying (I suppose) an disallowed action.  The boxes in question are not part of a slide background.  This problem does not occur with pictures.

I've found a workaround which is to select the box differently, namely by dragging the cursor around the box with the mouse button held down.  Then I can (sometimes) delete or copy it.  Still looking for a real fix.