This just started happening to me. I'm uploading via XCode 10.1.
For a while, I had a failed build, 53 for my app, that I could not get rid of in App Store Connect. It would not display link as a link in the builds list, so I could not get to the Expire Build button for it.
Build 54 turned up in Test Flight okay, but 55 and 56 have not.
So I decided to re-upload the failed Build 53 and let it process. This finally made it linkable in the App Store Connect builds list, so that I could then hit the Expire Build button on it. But this hasn't fixed the issue.
Currently, Build 54 is the hightest one showing in Test Flight, even though I've expired that build too in App Store Connect. I've subsequently uploaded a Build 57, and I also increased the App Number on that one, but still Test Flight shows the Build 54 as the highest available test build, even though it's expired on App Store Connect!
Very frustrating. I have no outstanding "Compliance" messages on my builds. I also checked the Developer Login to see if there were some updated Apple Terms & Conditions that I had to re-sign, but there were none.
Update - working now
What did it for me was to go into XCode and change one of the Entitlements on my app. I turned on iCloud Access, and then immediately turned it off again. I then rebuilt the app and resubmitted it, with yet another build number. Finally, it turned up in Test Flight.