Ciao Najma.Have you checked from your menu if the device is still linked to Strava? I mean if the related checkbox is flagged on. Look at the print below referring to my Garmin

Yes, it is on. And in Fitbit app, shows Strava in a list of connected applications.

It is weird that they are not connected.

Hello ​@Najma - sorry to hear that you are experiencing trouble syncing. Fitbit connection and 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.
 

Please follow the link to connect your Fitbit and Strava accounts and your activities will sync from Fitbit to Strava. We typically suggest reaching out to Fitbit directly since this is an issue on their end that we cannot look into. Hope this helps!