Similar issue for us,
for one of our apps it works fine, for one other, our test and stage apps works correctly but once in production mails are not delivered, or only a fraction of them.
No errors, logging says it worked.
Is there some throttling applied? The main difference with our production app is that it sends many more emails, about 17k per day.
The other production app on which it's working sends between 5k and 6k per day.
Our mails are signed using the new feature (DKIM for Google Apps sender), same sender between our different stage apps.