Why the leaderboard does not save players?

Make sure your campaign status is Live and your registration form has a Leaderboard Name field.

Is the campaign Live?
When your campaign is in Draft status, no data is saved. Draft status enables to quickly edit and preview the overall structure, but does not fully reflect end results.
If you wish to test how everything works, go Live by Publishing the campaign. You can always reset all the analytics after testing.

Does your registration form ask for Leaderboard Name?
Leaderboard results are displayed based on Leaderboard Name field input - without this data it does not show any results. Go to your registration form settings and check if this field is asked.