Creating custom Assignment Notification

I'm trying to create a way to send out assignment notifications for the Opportunities module only. One way to do this would be to override the send_assignment_notifications method, but I'd rather not do that. Instead, I'm keeping the system-wide assignment notifications off, and creating an Advanced Workflow that would do this. But, it doesn't send notifications. The process is shown in the image below.opportunity assignment notification process

The start event is set up like this:

start event

The Receive Message event is set up like this:

For some reason, no notifications are sent whether I create a new Opportunity or change the assigned user in an existing Opportunity. The process doesn't show up in the list of open processes. 

Any suggestions on what the problem might be would be appreciated!