If no ride ever uploads, and you’re connected to the internet, FulGaz isn’t actually connected to Strava. Reconnecting again usually fixes this. There is also a "test connection" button to verify your connection.
If some rides upload but not others, it will be the time it is taking to upload that is causing the issue. If the ride took you five hours or so to complete, or you’ve got a slower internet connection, the server can sometimes “time out” before the entire ride is uploaded. It’s worth checking that nothing else at home is slowing down your upload speed. This can sometimes be things like Dropbox or Google Drive.