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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
Note that fitbits only have enough memory to hold detailed data for up to 7 days between syncs. After that they start holding the data at a daily summary level. So, when you get your fitbit syncing again you may find that your data looks a little strange.
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