To check why an invoice form isn't integrating, in the Web Portal go to Forms > Form Integrations > Logs. Search for the Form template type that you're looking for (if you have more than one template that has integrations attached to it), and in the list you will be able to find the forms that have failed to send to Xero, and the reason why.
Once you've made the correction to the specified form(s), then simply Submit them again to push them to Xero.
