The photos must have a clear separation between them. If the images have white (light skies or snow) around the edges, place a piece of dark paper on top of the images in the scanner.
Try putting the photos more crooked on the scanner bed, and if they're very faded and pale, try putting a contrasting background behind them before you scan them. Also try a different arrangement. Something is preventing PSE from seing the edges properly.
Incidentally, many scanner drivers have the ability to do this these days, sometimes better than PSE can. If you strike out again with PSE, see if the scanner can do it for you.