I am using the platform Ushahidi Web-2.7.3 , see: http://ti5.net.br/provedorlegal.com.br, and when I zoom in beyond a certain level, the clustered markers disappear from the map. I also tested this on an older version of a site, see: http://movimentofichalimpa.org/mapa, where the clustered markers do not disappear on zooming in, but just become ungrouped, as is normal with a cluster strategy. How can I make the markers remain on the map when zooming in?
Ushahidi actually uses OpenLayers under the hood -- the images may come from Google, but the Javascript library is pure OpenLayers. These markers are actually generated by adding what is known as a context to the style attribute of the OpenLayers.Layer.Vector and an associated Cluster.Strategy. There is a good example of it working properly here: OpenLayers cluster example and if you look at the Javascript source you will see how it is done: view-source:http://dev.openlayers.org/releases/OpenLayers-2.13.1/examples/strategy-cluster-threshold.html by defining a radius in the context of the styleMap.
The reason that your example isn't working is that there is actually a script error being triggered, which is stopping the clustering/styling from being performed. The styles are defined in the file ushahidi.js. If you open a Javascript debugger you will see the error for yourself that is firing before the ushahidi.js functions are called to determine the number of features in the cluster and therefore the radius of the markers.
The actual error is http://ti5.net.br/provedorlegal.com.br/index.php/json/cluster?s=1401591600&e=1404183599&z=9, but I have no idea why, as the OpenLayers is the minified version which is very hard to debug. You might find moving your application to OpenLayers directly will help, as it will be easier to debug, and it also works on mobiles, which I know is one of the reasons people like to use Ushahidi.