Hi ​@Dennis Sw, we might need more details to be able to help.

Anyway, I’d recommend that you start here:

https://support.strava.com/hc/en-us/articles/216919177-Strava-Challenges
 

… and I’d highlight this -

• For your activities to count towards the challenge leaderboard results:
  • Eligible activities must be visible to everyone. 
  • Eligible activities need to meet challenge goals or requirements.
  • Manual activities will not count toward challenge leaderboards.

If the article doesn't help, please include more details - ideally include a screenshot of the particular challenge settings (showing information about eligibility), as well as screenshots or links to some activities that weren't counted but you believe they should have.