Sometimes when writing a LaTeX document with 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.