I forgot to add that I'm using JBoss 3.0 and I'm deploying the EJB from JBuilder 7.0.
Can you send us your "ejb-jar.xml", "jbosscmp-jdbc.xlml" and "postgres-service.xml" ?
I found the solution. The problem was the table already existed in the database (I had already created it). When I removed the table and redeployed the EJB, the table was created automatically and it works.