ios – TestFlight: Why isn’t individual tester getting notifications when new TestFlight builds are pushed to store?

I have an app on TestFlight, and I added an individual tester to version 0.1.3. I then pushed versions 0.1.4 and 0.1.5, and although this tester automatically appears under “Individual testers” for these last two builds, they didn’t receive any notifications about them, so they still have only version 0.1.3. They have auto-update set to ON. Does anyone know how I can resolve this?