Sorry about your syncing issues, @Cmhjgh. But I'm glad you found us here. You have correctly tried many parts of the the "fix," but it is important to do them in the right order. That said, please keep in mind that a good part of the problem - for Fitbit and for other apps - is that Apple keeps releasing different versions of IOS 10 and 11, forcing everyone else to play catch up as quickly as they can.
First, make sure you are on the latest update for the Fitbit app. Then, remove your Fitbit device from the Fitbit app and delete the app. Go into your iPhone settings. Select Bluetooth and Forget the Fitbit device. Then, make sure you are on the latest IOS version.
Wait a few minutes then reinstall the Fitbit app and set up your device again. It will automatically ask you about pairing it to the iPhone's Bluetooth.
This has worked for others. Hopefully, this will do the trick for you. This process gets all the updates in sync with the others. Please let us know if this works.
