System Workflow Extension won't Transition State

I am working on a Field Services customization which adds an extra state to the appointment workflow for a particular service order type. The state, called “Ready to Bill”, sits between Completed and Closed. I have an action defined in a graph extension of FSAppointmentEntry which runs some custom billing logic. The intent is to have users run this action before they can close a completed appointment, so that the detail lines are updated based on our business rules and the charges are all correct. The action itself is working perfectly.

The problem I have? No matter what I do, I cannot get the UI button for the action to actually transition the state of the appointment. I followed the instructions in the open university course W150 and the wiki:

1. Extend System Workflow
2. Add new state
3. Add existing action (picked my action, defined in code, from the list)
4. Add my action to the Completed state
5. Add transition to the Completed state

The instructions just end here, as if that is all you need to do, but I am obviously missing something as no state change is occurring even though I have proof of the action running my code successfully.

I’m at a bit of a loss as to what to try next, so if anyone has any ideas I would be most grateful!