Hey @Nean83, 
Thanks for your post and sorry to hear that you are experiencing trouble syncing your walks over to Strava. Fitbit syncing issues are unfortunately something we do not have visibility into. Fitbit actually pushes activities to Strava and if there is an activity on Strava that's not on Fitbit, Fitbit pulls the activity from us.
 
Fitbit normally recommends the following in troubleshooting syncing issues:

1. Try reconnecting your accounts, syncing your Fitbit tracker, and waiting an hour.
2. Confirm that you are not recording a duplicate exercise in the Strava app and Fitbit app/device simultaneously.
3. Confirm that you are recording a GPS exercise if using Fitbit (only exercises with GPS on Fitbit will be sent to Strava).
4. Confirm 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.

To upload the file exported from Fitbit to your Strava account, please click here:
 
If this doesn't work, we suggest reaching out to Fitbit directly since this is an issue on their end that we cannot look into. 

My run didn’t sync automatically but I did this and it is now on strava, hoping the problem sorts itself out next time and it starts syncing automatically again

Fitbit activities need gps data to auto-upload to Strava. For Fitbit users, go to your exercise screen on your watch, select walk, scroll down and turn on gps data. Once gps data is enabled, your future walks will auto-upload. Cheers!