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The usual things to try when you get syncing issues are:
If none of those ideas work for you then it’s probably best to contact customer support via: contact.fitbit.com
You need to get this fixed quite quickly as fitbits only have limited memory to store data between syncs. Depending on the model it can hold detailed data for up to 5 - 7 days after which is starts holding data at a daily summary level.
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