Summary
There are two scenarios when you complete a Work Order and it doesn't create an invoice.
Key Points
[ ] No charge (Callback) is checked on the Work Order
The Billing Frequency is different from the Scheduled Frequency
Make sure the check box for [ ] No charge (Callback) is clear on the Work Order. Callbacks do not generate an invoice.
If the check box is clear it's probably that the Billing Frequency is split from the Service Frequency. For example, it is generating an invoice every month rather than when the Work Order is completed. To fix this:
Open the customer's account page
Click the Service Location tab
Click the Work Order tab and find the Work Order
Click Edit Series just above and to the right of the Work Order line
Update the Billing Frequency to For Every Work Order
Click Update Series
Create a new invoice for the Work Order in question, (separately) as the system won't recreate it automatically