Can you post your expression script and a couple sample lines of data?
This may be because of heavy calculation, or your memory is full. In the initial days i was having similar sort of problem see the link http://community.qlik.com/forums/p/17478/68652.aspx#68652
1) sum(if(Time< $(TimeValue),PassedStatus))
2) ((sum(if(Time< $(TimeValue),PassedStatus,0)))/(sum(PassedStatus)))*100
Variable TimeValue can be 1 0r 2 0r anything based on my selection in slider.
Data Handled is like this
if(Status=Pass,1,0) as PassedStatus
Time from ....where exists (ID) ;