Ciao ​@Tanja. Have you checked the privacy settings of the activities not showing in group stats? Maybe you have activities saved as private.

Hi Vincenzo, thank you, but everything is set to public.

Hi ​@Tanja, you might need to check the activity types for that particular club, verify that some activity wasn't flagged etc - here's a more detailed article on this topic: https://support.strava.com/hc/en-us/articles/360017705031-Why-aren-t-my-activities-counting-towards-a-Club-leaderboard