Values in total and subtotal cells cannot be updated in the following cases:
When the cells contain blank or zero values. Numeric values are the only type that can be updated in total and subtotal cells.
When the Others Summary property is set to Yes on the measure items axis.
When the cells are on the same axis as measure items. Only total and subtotal values on the axis opposite the one for measure items can be changed.
When some members on the axis opposite the measure items axis are not visible. All members of the result set should be made visible – you can make all members visible by disabling the Use Paging setting in a view’s Display Options or clicking Show All in the view toolbar. Views that fit on a single page whether or not paging is in use also meet this condition.