Hello:
The first thing to understand is that, once invoices are posted, there is no link between what you see in Adagio Invoices and what you see in Receivables. It is quite possible, even likely, that your Invoices history will stay on file longer than your AR history.
Secondly, do you run AR "Period End"? Do you place a check-mark on the option labelled "Clear Customer history up to 00/00/00"? If so, all History transactions prior to that date will be purged. These are transactions that are already paid and already moved from the "Current" list to the "History" list.
What is odd, however, is that some Customers have 2004 data while others don't. Do you ever run the "Clear History" process under the "Maintenance" option in AR? This option allows you to clear History for a range of Customers. Is it possible that someone has run this process on at least the Customer in question?