If your Facebook Conversion API action in a workflow doesn't seem to be working, first check how you're testing it. The test workflow button won't send a real conversion, so please don't use it to test the Facebook Conversion API.
Instead, always save and publish your workflow. Then test your conversion using one of the triggers below.
The trigger needs to be an input trigger, such as:
Form Submission
Survey submission
Appointment Submission (from booking widget)
2 step order form
If you are using an appointment trigger it will only work with the 'Customer Booked appointment' trigger. It won't work with the general appointment trigger. That's because the general appointment trigger covers all appointments, while the Customer Booked appointment is the trigger for the booking widget.
Lead value: if you're using the Purchase event, the lead value is required. For any other event, you can leave it blank.