Google Maps Plugin Marker Cluster Not Working

I have a sample map at regenexx.com/sandbox with two Chicago locations displayed that sit on top of each other and make the second location difficult to spot. I activated the marker cluster, but nothing is happening. It showed up for me once a while ago, but was intermittent. Now I can't get it to appear on a regular basis.

Can you take a look and see if you see a problem?

Thanks,
Dave Klein
Klein New Media