Q: [?] instead of bib ref

[Laurent DUVAL]

Hello,

Sometimes I get [?] instead of the correct bib refin my whole document
, even though
the Bibliography at the end of the document is correct.
Each time I get this kind of trouble, I perform random actions such as
close the file, rename the bib file, delete the /tmp file.

Sometimes it works, sometimes it doesn't, but I can never remember 
what is the correct type of action 
(my actions are genuinely random, great!).

Could somebody give me a hint? (lyx1.0.1, Sun Solaris)