Q: mighty mouse won't open dashboard

For the last year or so, my mighty mouse no longer opens dashboard or any other program as it did when I first got my iMac.  It scrolls just fine, and the left and right buttons function perfectly.  I have gone into settings several times to try to correct the issue, disconnected/re-connected the mouse, etc, but the problem persists.

Any suggestions of correcting this?  I rather like the ability to open dashboard by clicking the scroll ball.