BCC recipient information is stored in non-standard email headers. POPcon searches for this information in the X-RCPT-TO, X-Original-To, Delivered-To, and other header fields.
If BCC delivery is failing, the intermediate mail server (your ISP or the sender's mail system) may have stripped the BCC headers before POPcon received the message. In that case, there is no way for POPcon to determine the BCC recipients.
If you believe the headers are present but POPcon is not detecting them, contact support and provide the full email headers for analysis.
Why BCC works with individual mailboxes but not always with a catch-all mailbox is explained in the guide Catch-all (multidrop) mailbox vs one POP3 mailbox per user.