Make sure that the Auto Sync works in the background!

How? Follow these steps:

Enable Background App Refresh:

• Open Settings App
• Scroll down and tap on "FitbitSync”
• enable “Background App Refresh”

Check Apple Health set-up:

• Open Settings app
• Tap on "Privacy"
• Tap on "Apple Health”
• Tap on “FitbitSync”
• Turn on all

Check Location Access:

• Open Settings App
• Go to FitbitSync
• Tap on “Location Services”
• Tap on Always 
   
• Go back in the Settings of your iPhone
• Go to “Battery”
• Deactivate “Low Power Mode”

Please note: it is necessary to always have FitbitSync and Fitbit open in the background.