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.

I am now using Pack and Track AP satchels which are more expensive simply because I found the entry process for Click and Send frustratingly slow and because I only have a printer connected to an office computer not the one I do most ebay transactions on. 

However I am pretty sure it was a Click and Send parcel that I had a not received enquiry on some months back and by entering the tracking number I was able to advise that AP had attempted delivery and had returned the parcel to the P.O. as nobody was home.  (Apparently no card was left at the address but the item was indeed awaiting collection at the post office).