Sometimes when writing a LaTeX document with Emacs.app in MacOS and I
tried to put citation in the document by typing Ctl-C + [, emacs fails
to search references I am looking for. I do not know the reason exactly,
but I seems that emacs does not store the reference information in it.
So the solution is to reload the reference file by typing
M-x Reftex-parse-all. Then emacs will reload bibtex database and
reftex by Ctl-C + [ begins to work again, perhaps.