App Store Server Notifications not always sent to my server

We have implemented an auto-renewing subscription as an in-app-purchase for our iOS application. We are consuming the App Store Server Notifications for subscription transactions in order to update the user's account (and thereby maintain their 'Pro' access to our application). Sometimes those notifications never come to our server, and there is no evidence that they were even attempted to be sent to us.

We have had some users report to our Customer Support team that they have successfully made a purchase of the subscription, but that they were not granted 'Pro' access.

For the large majority of users this is not happening and all is well, but for some users the notifications just never come from the App Store Server API.

We keep a record of all notifications that we receive from the App Store, and for these users we never received the "SUBSCRIBED" event. We have checked the Notification History API and there are no reports of any failure to send notifications to our server.

We have checked our server logs for any sign of failure to receive incoming web requests, and there is no sign of these missing notifications. We have verified that our server supports ATS.

We are keeping the transaction.originalID for all our users who are subscribed to the auto-renewing subscription. We have used this value to do some lookups into the transaction history and subscription status of the users who are being affected. Here is an example of our findings from those lookups:

• From the transaction history endpoint, we received an error: “Invalid transaction identifier”.

• From the subscription status endpoint, we received a response with the information for that user's active, valid subscription.

• We never received any App Store Server Notifications about this user’s subscription, and the transaction history API tells us it is an invalid transaction ID.

We believe that the fact that the subscription status API returned the information showing that this user’s subscription is active and valid, and that the notification history API shows no sign of a failure to send us notifications about that subscription, shows that the App Store Server API never attempted to send us any notification for this user’s subscription. The same is true for a significant number of other users of our service.

Can anyone help us determine what is going on, and how to best support these customers? It seems as though there was never an attempt to send these notifications to our server, but our users provide proof that they do in fact have an active subscription, for which they have paid (receipt email from Apple with a valid order ID).