I found a fix, not sure it will work on your case, but just try to see.
I think the problem is with the pairing system (bond) in Bluetooth and the
chip that monitors heart rate.
I was trying to install Fitbit app on iPad to sync. But I think the app is
mainly for iPhone.
Even someone else said that both iPad and iPhone could be used to sync,
there is interference for the rate to show up.
So, remove the device from the app then delete app, reinstall the app from
the cloud.
Open app, log in with fitbit account.
Turn on Bluetooth. Make sure no other Bluetooth connected device near by,
or interference will happen.
Set up in Fitbit app as a new device.
It will ask to be paired again, so follow the prompt.
It will be annoying to fix the UI settings to your likeness.
If while syncing/updating, you keep seeing the circling in progress takes
very long, need to Forget the device in Bluetooth settings, and re-pair
again.
Hope it helps. If not working, email me again, I will try to explain better.