TestFlight: Why isn't individual tester getting notifications when new TestFlight builds are pushed to store?
When you add an individual tester to a specific build, they're not automatically added to new builds you push to TestFlight. You need to do that manually - or add the user to a group.
Note that even if the individual tester is added to a specific build, it doesn't mean that it is available to them yet. You do need to ensure that you have entered the required encryption export compliance information, as well as having sent in the app for beta app review and received an approval.