It could be that you did not give permission in Strava to upload activities that are marked as private. What you can do is reconnect to Strava and make sure both checkmarks are enabled. After that, JOIN is able to read your activities and use them in making calculations. Obviously, another solution would be to change your Strava settings from 'private' to 'public'.
It could also be that you have the wrong Strava account linked to JOIN, to fix this please follow this link for more information - https://help.join.cc/hc/en-150/articles/5440468599441-Logged-in-with-the-wrong-Strava-account-