Hi Din,
This may be because projection function is not set in universe.Please set projection function to Sum in Universe, that will be used by Webi for aggregation.
In 3.1(UDT) You can find the option to set Projection function as shown below:
In 4.0(IDT) You can find the option to set Projection function as shown below:
Hope it helps!
Regards
Sheetal Sharma