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Unfortunately, syncing problems happen from time to time, often for no apparent reason.
The usual things to try are:
If none of those ideas work for you then it’s probably best to contact customer support via: contact.fitbit.com
Hopefully you'll get it fixed soon as fitbits only have enough storage to hold 5 - 7 days of data between syncs. After that they start to hold the data at a daily summary level rather than the usual detailed level.
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