Are you properly identifying the primary key in your Hibernate entity (simple or compound)? If you don't, Hibernate won't know what it is. If you're using annotations, there is a specific annotation for that; if you're using xml, there is specific xml for that.
With best regards,
Anton Golovin (email@example.com) SCJP, SCJD, SCBCD, SCWCD, OCEJWSD, SCEA/OCMJEA [JEE certs from Sun/Oracle]