Steve,
There is a separate command for Stand-Alone symbol update. It is under the
"Cross-Ref" toolbar flyout. It has the "Stand alone" checked and the "wire
update" unchecked.
Alternately, if you want to try to update a stand-alone symbol by just
picking on it, try this call:
(wd_sig_do_sigs 0 1 nil (list (car (entsel))) nil)