In order to troubleshoot mailing list providers issues with WP Contest Creator you need to have WP Contest Creator v1.4.0 or greater. You can check your current version by going into your Contest Admin then go to the "Settings" tab.
To troubleshoot opt-in provider issues:
1- Scroll to the bottom of the page.
2- Click on the checkbox labeled:
Enable Debug-mode for opt-in provider (to troubleshoot opt-in provider errors only, disable when done)
3- Go back to the contest page (front-end) you are having issues syncing your leads with your mailing list provider.
4- Do a test by filling out the contest form you created and follow all steps as a regular lead would do when signing up to your contest.
5- Once you submit your test lead, go back to your Wordpress Admin panel.
6- Go to: Contests -> Settings and scroll to the bottom of the page.
8- At the very bottom of the page there is a textarea below the checkbox. This textarea will be populated with the HTML error page from you mailing list provider after you submitted your test lead.
Once you see the details of the error message, make the proper changes in your validation rules on the mailing list provider form fields or adjust the custom fields on WP Contest Creator so the values passed from the contest form are accepted by your mailing list provider form.