Answer 1

There's really nothing much you can do if a user declines your challenge. But if some users accept your challenge, challenge them back as soon as possible. That way, there's a high chance that they're still active and can accept/decline your challenge.

If you want challenges, you can post here:

https://www.sololearn.com/discuss/502545/?ref=app
https://www.sololearn.com/Discuss/864805/?ref=app