Are they truly being “asked to pay again” or just being returned to the registration page. Returning to the registration page is the default. You can change that by using a success=""
attribute in your shortcode to create a custom “thank you” page. See this KBA for details: https://s2member.com/kb-article/configuring-custom-return-urls-thank-you-pages-upon-success/.
To clarify the question I’m asking: are they being registered as Users? Check WP Dashboard → Users.
If they are being registered, but not receiving any emails from your site you’ll need to determine whether the emails are being sent and not received or are not being sent. To do so, you’ll need to install a plugin to log emails sent from your site. If you just need a logger, I’d suggest WP Mail Logging but there are plenty of plugins that do the trick.
Once you’ve installed the email logging plugin, test your registration form again. Check the email log to see if the email was sent. If it wasn’t you’ll need to do some deeper troubleshooting. Let me know if this is the case and we’ll go from there.
If the emails are being sent, but are never arriving you are probably running into problems where the emails are being discarded as spam at a level before the User’s mailbox. Transactional emails from shared webhosting servers are especially prone to this. So, if your emails are being sent, but never arriving I’d suggest a transactional email service. I use SendGrid.