I may be missing something obvious here but I have not seen any explanation for the missing parcels.  Using Click and Send the tracking number is automatically uploaded to the ebay posted entry so even without an AP receipt the item should be trackable by entering the number in the AP tracking site.

It is only trackable if AP scans it; if it is not scanned how do you imagine it can be tracked?  When you enter it on line the number appears in your sold items, but unless it is scanned when you hand it over there is no proof it was actually posted.