Report not displaying invoices

I'm running QB Pro 2005 and attempting to run a "Sales by Customer Detail" report. In a typical report I get two lines for each invoice, one showing the invoice amount and another sales tax line (which has no amount because the customer isn't taxable). But when I attempt to filter the report by invoice number, I only get one line, the sales tax line. Any ideas on what might be causing this? I have created a custom field that shows data on the line that isn't sales tax, but that field is empty (thought the column is there) when I filter by invoice number.