Hi there @Mark31, its nice to have you in the Fitbit Community! If you are finding troubles to sync your Flex 2 with your phone, perhaps the following post might get you back on track: Having trouble syncing? (here's how to fix this).
Make sure your phone is compatible and you are running the latest version of the app for Android device (The last version is 2.45). You will find this recommendations too in the suggested post.
Give it a try and keep me posted how it goes.
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