The "Acknowledge" function for an alert isn't working

I set up alerts, define escalation schemes, and tell the scheme to send the alert to the role "Super User", of which I am a member.

The alerts fire, I see them in the  banner header, in the Recent Alerts portlet, in the Alert Center, etc.

But, there is no "Ack" button anywhere.  In the portlet, I see the "acknowledge" button, but it's greyed out.

Does anybody know why?  What am I doing wrong here? 

Thanks!