Have uploaded a new build on iTunes Connect using Xcode 8. The build is showing under activities. But no build is showed under TestFlight IOS Builds. Event the old builds which were distributed earlier using testflight are not showing up.
The page for External tester and iOS Build loads no information.
I'm also having this problem...
After the uploaded build was processed, there was an option for me in the app store window to add the build, and after adding it there, it started appearing in the test flight as well.
Restarting my iOS device (phone) helped me load the latest build from the TestFlight app on my device - hope this helps anyone stumbling across this in the future!