If a viewlet is empty (displays the message, No data to display), Administrative users can check if the objects' node is managed. Please note that when a node appears as managed, sometimes un-managing and re-managing the node might be needed to make the node active. Go to How do I enable or disable the management/monitoring of a node? for instructions on how to re-manage a node.
TIP: Please see the article, Objects are missing from a viewlet, for other reasons why a viewlet might be empty or missing objects.