This may be due to an overly narrow time window (that is, no logs were sent in the last 15 minutes, which is the default time window). Try increasing the time window with the drop-down box in the screen's top right corner. If the problem persists verify that log agents are running properly; make sure the openstack-monasca-log-agent service is running and check /var/log/monasca-log-agent/agent.log for errors on the agent nodes.