Susan:

There is no known program reason for the AR Trial Balance to be out of sync with Ledger.

There is a long list of user errors that could result in the AR balance being different from the Ledger balance.

To start; make sure you are using the AR Aged Trial Balance with the Calculate as of date option.

Make sure AR balance to GL as of this month end, so make sure that all transactions are posted to Ledger, print the report for May 31 and compare the balance to Ledger for this period end. In sync, then you have a timing error to find. Not in sync then you have a bigger problem.

Timing errors occur because someone overrides the automatically assigned fiscal period based on the transaction date. This happens often at year end when someone decides that transactions dated in one fiscal year (like the last) should be moved into the next fiscal year. Now you AR is out of balance to Ledger at year end.

Beyond that it will get difficult to guide you through the reconciliation process in this forum.