Hi Madhavi,
In which section you place the sub report in main report? If you place the sub report after report header section then it will process after the report header section. that is the reason it is displaying blank.
--if you would like to get the values from sub report, then the main report would process the sub report before the report header section.
--So you need to add one more section for report header section and put the sub report on top one and place header values on the second one.
Hope this information will help you.
--Naga.