I have fixed this error. This error will happens completely based on your plugin or RHQ Server Console.
Your Plugin :
If it is your plugin please check out agent error log. Because It could be anything.
RHQ Server Console :
Please check Schedules > tables tab;
In that tables tab, If you does not receive min , max , avg values, But you are receiving values in Last.
Please check BaseLine in RHQ.( In my case, this is the problem.)