problem while referencing in the text, appears as (?) in the pdf and dpi format

[marcela palomino]

Hi, I can't solve this problem which is becoming quite frustrating.
I'm using JabRef to reference in my thesis, using Natlib and agsm (have 
tried other formats too). I need to have the (author, year) format 
in the document.
When I do it, looks fine in the LyX version, but when I convert it to PDF or 
PDI files, it appears as (? ), although the reference list is perfect.
Can't find the reason, and would really appreciate any help.
Marcela