This problem is fixed in versions after 4.0.11.
This occurs when the last customer alphabetically is due within the report period. Any customer due after the last customer will not show up on report.
To correct this problem in versions prior to 4.0.11 do the following.
Go to Point of Sale > Retail Sales.
Type ZZ in the name box and push F6, then ESC back to main menu.
This will create a fake last customer which will never be overdue, thus preventing this error to occur.
Or just obtain a new version of RTO Pro after 4.0.11 and the problem will be fixed internally.