Export to excel - row grouping with calculation does not update value when cell content is updated.

Hi,

I am using SSRS 2008 Version. In my report i have grouping and calculations (eg: sum() ) done at row level. Report works fine. When I export the report to excel  and modify the value of the cell (as data needs to be updated) value, the sum()  at group leavel is not updated and  it still has previous data. Could you please explain what can be done to fix this issue?

Thanks,

RK