Approval Emails stopped sending mid-process

We're finally ready to start using approval processes, so I've been creating some tests to demo it for key people.

Part way through the process WHD stopped sending approval emails. Steps 1, 2 and 3 sent emails to the approvers/CABs, but Step 4 did not. So my test helper approved Step 4 through the web interface... but then no email for approval step 5. So I did an override, and then, again, no email for Step 6. There is no entry in the ticket history stating it even tried to send the email, so it's not an email issue at all.

Has anyone run into this?