Hello davidcarteruk6-g, thanks for joining the Community
This happens when any credits on the customers account have auto-applied to the invoice as payment.
To check and edit your automated settings, go to the Settings wheel > Account & settings > Advanced > Automation > Automatically apply credits. From here, you can toggle off the settings (move the slider to the left).
To remove the payment from the invoice, open the invoice and click on 1 payment made > select the date > on the Receive payment screen, click More > Delete. This will un-link any credits so that it can be applied to a future invoice.
