no longer able to drag and drop to edit pdf

I have adobe acrobat 11.0.06 and Mac OS 10.9.1.  I use to be able to add files to an existing pdf via the "Page Thumbnails"view. I would drag the files and drop them there and "bingo" the pages would appear where I dropped them and in the correct order.  Now I cannot.  Now, only the last page of the file I am dragging will appear.  I can add the files using the tools>Insert pages>"Insert from file"  .