Hi Lamour,
The due date I believe is when the officer handling specific areas of ones files needs to have them completed, so that it can move along the lines to where it needs to be finalized. During my kids application, they had gone past all due dates by a very long time and since he was a minor we decided to involve our MP. That helped tons.
Unfortunately with AVO it's mostly the case, one or more officers inability to effectively perform their duties with zero follow up from supervisors tends to be one of the major causes of SA delays. That's where our prayers come to play. Most frustrating I must say! But hang in there! *hugs*