Here are some issues we’ve come across that cause this to happen:
One of the best tools for searching for errors in the HTML page headers is to use Firefox with the Firebug plugin. Turn on the console and check the responses tab for any errors. We have resolved many client installation and map problems with this plugin. More often than not we are finding errors in other plugins, themes, or server configuration settings on the servers. No, our plugin is not perfect and is not 100% bug free (we wish it were) but we are no longer finding bugs in our plugin as the primary culprit in broke installations.