[OS X Emacs] backwards search with aquamacs and Skim

[Arthur E. OGUS]

I am working on a large mathematics manuscript, and suddenly the PDF synchronization with Aquamacs and Skim is broken.  The forward search (from the emacs window to the Skim window) works,
but  not the backwards search (from Skim to Aquamacs).  Command-shift click does nothing.  It still works on my laptop, but not on my desktop.  There are many places this could go wrong; any suggestions
on how to track it down?

Thanks.