Please confirm you've done the following:
1) Tried reconnecting your Strava and Fitbit accounts, syncing your Fitbit tracker, and waiting an hour.
2) Confirmed that you are not recording a duplicate exercise in the Strava app and Fitbit app/device simultaneously.
3) Confirmed that you are recording a GPS exercise if using Fitbit? (only exercises with GPS on Fitbit will be sent to Strava).
4) Confirmed that you recorded a new exercise after connecting your accounts. (Previous exercises won’t be uploaded. Athletes can use the TCX export from Fitbit to upload older exercises manually.)