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Usually, restarting the phone and fitbit sorts out syncing issues for me. But when they don't I have had success in "forgetting" the link to the fitbit in your phone's bluetooth settings.
If that doesn't help then you could try Fitbit's troubleshooting guide at Help article: Why won't my Fitbit device sync? and, if that doesn't help, then it's probably best to contact customer support via contact.fitbit.com
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