My Fitbit won't sync

Go to fitbit.com and log into your account > Settings > Applications > OWHEALTH Period Tracker > Revoke access.

Then, register your account in Flo Menu ( or your avatar) > Profile (this is an additional step for Android users) > Continue and reinstall the app.

After that, log into your Flo account and enable Fitbit via Settings > Enable Fitbit > Log into your Fitbit account. You may have to wait till the next day (as Fitbit syncs with Flo only once a day) to check if the problem is resolved.