I have been stung by non-payment in the past, it’s heartbreaking when you’ve put so much time and effort into the visit and report. Have you followed up with a polite reminder that the invoice/payment is outstanding? Did you define payment time frame?
Changes I have made following on from my experience include now asking for payment of initial assessments upfront in order to secure the booking, payment after the appointment is then available for therapy sessions. I am now also very clear on my T&Cs and define a payment period on my invoices so that it is clear. Something worth considering in the future as a safeguard.