May be you are having discreet measure so Grand Total won't show. Can you post a workbook?
Grand totals may appear as null for multiple reasons and the exact solution can be suggested only after a look at your workbook.
For example: if you are using attr to aggregate your measure you could see NULL try min() instead. Check this: Grand Totals Are Blank For Calculated Field That Include ATTR() Including Fields That Blend Data From Multiple Data Sour…
Or for more detailed understanding of grand totals check : Customizing Grand Totals – Part 1 | Drawing with Numbers
Hope this helps.