Are you setting both properties in the relationship, so calling the setter on both the owned, and the owning class? I'd bet that is not happening.
Also, have you set up a cascade type for the relationship? You might want to look at setting up a CascadeType.ALL setting somewhere, although I'm not sure if that would help in this particular scenario.
Here's a helpful tutorial on object relational mappings from Wikipedia:
http://en.wikipedia.org/wiki/Object-relational_mapping
Also, I have some free tutorials on implementing simple many-to-one mappings if you're interested:
http://www.hiberbook.com/HiberBookWeb/learn.jsp?tutorial=18mappingonetomanyassociations
-Cameron McKenzie