Approval Process Not Firing

So I set up what I thought was a pretty straightforward approval process. 

When a salesperson sends an invoice, it first must be approved by a sales manager, and then it's sent. 

An invoice was sent but the manager did not get the email saying he needed to approve it and I'm not sure what went wrong. When I tested it, it worked fine.

I have attached screenshots of the approval process for more information.