Reason could be the DistributedPolling setting is not in effect for the Invoice DB adapter; try re-configuring/recreating the adapter. Also, double check whether primary key is correctly set for the Invoice table in the DB adapter so the adapter can uniquely identify a record. Also, ensure that the after read configuration for polling is correctly set (say, logical delete) so the records once processed are marked.
thanks for the response. I'll reconfigure the DBAdapter and try it again.