Check for the following reasons:
Click on the current date in the Calendar. If data is displayed, then there could be some time difference between Admin and Collector Server.
If both Admin and Collector Servers are in different time zones, then you need to choose the appropriate time using Calendar.
The possible reasons could be:
The Admin Server unable to contact Collector Server or the Collector Server status is down.
a. If the Admin Server is unable to contact Collector Server,
b. If the Collector Server status is down, check for the following conditions: