Have you checked the servers error logs for any errors around the same time as when the registration(s) were made?
Users are reporting registrations issues, They go through the process but something fails at the payment page and they show as an abandoned booking, they’re not though, it failed.
Depending on the error, EE may not have any way to know that the payment failed.
If you look in the transaction do you see a failed payment there?
The transaction status is set to abandonded as soon as the user adds the attendee information and opens the payment options, this is because at that point EE doesn’t have any additional info, the user may be continuing through the process (in which case the transaction status will update) or they’ve closed the browser, but EE can’t tell which is which at that specific time.