If you set the following parameters in your jbpm.hibernate.cfg.xml
<property name="hibernate.format_sql">true</property>
<property name="hibernate.show_sql">true</property>
<property name="hibernate.use_sql_comments">true</property>
then you can see the query being generated by jBPM and work out why your task isn't being loaded by running it directly on the database.
I had a similar problem and it turned out that it was because my task had never been persisted.
(spring configuration mistake on my part).