Do you have any products ignored? From the comments in the language file, pay attention to the bolded portion...
Possibly. The report does nothing with session settings, and this issue has been reported many times by many users across different pages in the Admin. Do some forum searching to look for possible solutions.Code://////////////////////////////////////////////////////////
// EXCLUDE SPECIFIED PRODUCTS
// Prevents specified products from appearing on the sales
// report at all. **ADDING PRODUCTS TO THIS DEFINE WILL
// IMPACT TOTALS CALCULATIONS!**
//
// The value of the product will be excluded from totals
// for gc_sold, gc_sold_qty, goods, num_products, and
// diff_products.
//
// The values for gc_used, gc_used_qty, discount,
// discount_qty, tax, and shipping all come from the
// orders_total table, and so CANNOT be excluded based
// on product ID.
//
// If an order is made up entirely of excluded products,
// and has no shipping, discounts, tax, or used gift
// certificates, it will have a total of 0. In this
// situation, the order will not be displayed in the results.