“We never received your invoice.”

The invoice is missing from their system
A flooring installer emails an overdue account to the site manager. The manager says accounts never received the original and asks for a fresh invoice. The office is tempted to change its date to today. Before doing that, it needs to distinguish a delivery problem from a genuine error in the invoice, and preserve what was originally issued.
Keep the original transaction trace
Locate the first invoice, recipient address, sending time and any portal receipt. Save bounced messages and replies. An email in your sent folder is useful evidence of sending, but it may not establish successful receipt or contractual service.
Do not overwrite the original while investigating. If you resend an unchanged document, identify it clearly as a copy so the customer does not accidentally enter two separate liabilities.
Find the correct submission route
Ask accounts which legal entity, email address or portal receives invoices for this job. Check any purchase-order reference or completion document required under the arrangement. Confirm the person who can acknowledge that the package is usable.
Business.gov.au recommends an effective invoicing system as part of managing unpaid debts. Here, the practical improvement is a receipt check at the customer's actual entry point rather than another message to the site team.
Source: business.gov.au - What to do when you have not been paid
Separate a copy from a correction
If the invoice contains an error, ask your accountant how to correct the record properly. Explain exactly what changes and keep the relationship between the old and corrected documents visible.
If nothing is wrong, do not casually agree that the debt only began today. Ask for the payment position under the actual contract. Whether receipt affects the due date depends on the terms and applicable requirements.
Source: business.gov.au - What to do when you have not been paid
Get a processing answer
Request confirmation that the invoice has been entered, whether approval remains outstanding and the expected payment run. A reply saying received is useful, but it does not mean approved or paid.
If the account repeatedly disappears, review the workflow with the customer before the next job. Keep that operational discussion separate from any disputed entitlement on the existing invoice.
Resend once with a clear audit trail
Preserve the original and send an identified copy through the confirmed route.
Obtain the processing reference and payment position; seek advice before accepting a changed due date.
Call CRP today about debt recovery and coordinating the appropriate professional support.