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. 

The challenge is time based.

Surf is aloud for this challenge.

Does it say that it is measuring elapsed time or moving time?