New Payment name not allocating to invoice

[chown-root]

I have an issue whereby I create a new payment from the cutsomer invoice and the new payment shows the invoice number but when I add in the customer name from the search function the invoice number disappears and the drop down function shows "list is empty"
I have tried this with a new customer and the try and record the payment, I get the same issue.
I am now unable to record payments. I have about 40 customers and this has never happened before..