The partial sums may not display if they are zero-valued or null and you have the Suppress Zero Values option checked. You can verify this by unchecking the option and seeing if the partial totals appear.
Do you have any naked field references in the expression (ie not inside an aggregagation like Sum/Max/Count...)? This expression may be null for the partial totals and therefore suppressed.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein