Hello, @userfinance.
I'm pleased to guide you to the right support who can help you check why payment processed through Melio.
Since the payment was processed through Melio, I encourage reaching out to their Support so they can review the payment that wasn't received by your vendor. They can also guide you through possible options to pay your vendor successfully.
As always, feel free to read these articles to learn more about processing bill and bill payments in QuickBooks Online
Also, read the topics from this link in case you need additional guides while working with QuickBooks in the future.
Post again if you have any other questions. I'm always here to help you out. Keep safe, @userfinance!