Without seeing your example, below are the rules for turning on Grand Totals -

• The view must have at least one header – Headers are displayed whenever you place a dimension on the Columns shelf or the Rows shelf. If column headers are displayed, you can calculate grand totals for columns. If row headers are displayed, you can calculate grand totals for rows.
• Measures must be aggregated – The aggregation determines the values displayed for the totals. See  Grand Totals and Aggregations for more information.
• Grand Totals cannot be applied to continuous dimensions.