process needs to be put in place to deal with PENDING invoices which don;t get paid.
As has already been discussed in detail with support.
When an invoice is in "pending payment" status it will wait indefinitely for a callback from GoCardless.
If that callback never happens or fails for some reason, the invoice just stays in "pending" state forever, and no-one will ever be notified.
This means that either 1) the invoice never got paid or 2) the payment will never be recorded in WHMCS
But you will never know unless you manually checking for "pending" invoices.
A process needs to be put in place to handle these invoices if they have not been paid within x number of days.
So that they are switched to unpaid and the overdue notifications get sent or a notification sent to admins to check the payment/invoice for problems.