TestFlight says I have been notified of app, but app does not show in TestFlight App

We've had issues with TestFlight where a high percentage of our External Testers never get their email notifications when a new build is made available to them. Even though iTunes Connect says they've been notified, they actually haven't. We work around this by removing from External Testers and then adding them back using Add Existing Tester. But this is not ideal because we only know to do this when the beta tester has reached out to us to tell us they haven't received a TestFlight email. Hoping that Apple can improve the reliability of the TestFlight email notifications!

I have a similar problem, but with an internal user (me). On a new build that has been processed and added to Itunes Connect, then added to Testflight for internal testing (and shown as testing started), I get the notification email but clicking on the Testflight link brings up Testflight with a blank screen that says an Apple developer needs to invite me! I am set up as an internal testing user in Users and Roles in Itunes Connect. This is the first time this has happened over 3 tested and released apps. Only thing at all different is that I added an in app purchase capability to the app and added the in app purchase record to Itunes Connect.

This is a big problem. Can anyone help? Thanks!