Perhaps what you need to do is specify the scope of the RowCount function to a higher level of grouping than the immediate, default scope.
For example, suppose there are three levels of grouping on a table: Group A, Group B, and Group C, each one nested inside the previous group such that Group C is the detail grouping, Group B contains Group C, and Group A contains Group B.
If you have a text box in Group C and you have an expression of = RowCount(Fields!field_name.Value), you will get a value of 1 since Group C is the detail "group." But in that very same text box inside Group C you can have an expression of =RowCount(Fields!field_name.Value, "Group B"), and that will usually return a value greater than 1; it will show the number of rows in Group B that are grouped along with the current individual row of Group C. The same logic applies to = RowCount(Fields!field_name.Value, "Group A").
You can even have = RowCount(Fields!field_name.Value, "dataset_name") to return the count of rows in the entire dataset.
Hope this helps,