Double-check and make sure you've actually completed the challenge's description. Some challenges also take a couple of minutes to update properly. Make sure to reload your page when this happens.
You'll also notice that there are some levels with "repeatable" challenges. These challenges are usually identified with a yellow counter on the top right of the challenge's box (which also indicates amount of times you've completed the challenge).
The maximum amount of times a challenge can be re-completed is represented by another counter in the challenge's description.
If you're confident that you've completed a challenge, but didn't receive the correct amount of experience points (xp) for it, please submit a support ticket here so we can look into your issue.