As far as I'm aware there's no way to link it directly from FitBit (FB) to Garmin Connect (GC), but you can set up MyFitnessPal (MFP) as an intermediary. Have FB syncing weight to MFP, then MFP to GC. So double check weight updates are going from FB to MFP by checking both are up to date. If they aren't, get that fixed up first. If they are but it's then not coming to GC, break the link between the two services, make sure it's definitely disconnected, then re-connect the two services and that usually starts them syncing again.
