What is the current status of the customer?
Are they currently Suspended? Paused/On Hold? Billing will not continue until the customer is Active again.
What was the previous status of the customer?
Was the customer Suspended? Paused/on Hold? The customer or account's billing settings for what to do when a customer is removed from suspension will define whether or not it bills for any missed periods.
Is the account using Advanced/Avalara direct Taxation?
An invalid address could be causing a failure to generate invoice information. We have an existing custom reports to facilitate troubleshooting this issue. It can also be checked by simply re-adding the customer's current address information, if it fails, then there is almost certainly a problem.
Is there an unknown payment on the customer?
If there is an existing Unknown payment, and if Auto post AND Auto Collect are enabled on the customer. The invoices will not finalize, as the system cannot process the whole route at once. Payments will immediately be rejected (not fail) until Unknown Payments are resolved for that end customer. This can be potentially bypassed by temporarily disabling auto collect in order to generate the invoices. But the issue of being unable to collect the payment will still stand.