Work Item delay of 15 days not added after due date expires

Hi

We have set up our first work item 'Soft Call' so that when a invoice has become deliquent  the 'Strategy Wait Time Until it Executes' is 15 days. We have found that if an invoice has a due date of 30 days, that is invoice date 11 Nov 13, due date 11 Dec 13, the invoce will appear under the strategy work item 'Soft Call'. Our users were expecting that the system would wait a further 15 days on top of the due date of 30 days for it to became a Soft Call work items, that it it won't appear