Changing Sales Figures

We have a sales report that we now run at the end of the month to show sales totals for each store for that month. We have just run the July report and , because some shops had not uploaded all their sales data (we only upload daily and sometimes the phone lines are not avaialble!), we had cause to run the report again two days later. Using exactly teh same selection criteria and date range, some stores are showing a drop in the number and value of sales for the month! The drop is about 8 items. Its not happening in all stores but is happening in most. We cannot come up with any hypothesis to explain why the sales figures would fall for a period when the report is run on two consecutive days. Any suggestions for where we could start to investigate would be appreciated. Thanks Dave