I just got off the phone with PayPal and [after months of ignored support tickets] they finally acknowledged there is an ongoing error with PayPal PDT, and they say there is no timetable for a resolution.

According to PayPal Merchant Technical Support, they recently released an "upgrade" to the system that breaks PDT, preventing payment data from being transferred from PayPal's servers to your own securely.

So for all merchants using PDT, be careful. The system is no longer operating properly and is liable to break randomly. This is not to say it is entirely broken, but instead it will "drop" 1/100 customers from the records, as if they never completed a transaction, even though the money is taken out of their account.

As I said, PayPal FINALLY acknowledges this issue however they have stated there is no viable resolution in the near future.

So for those using PDT, be careful!