Certain payment records cannot be adjusted and therefore will not be shown in the Payment Adjustment lookup window. There are several reasons why a payment cannot be adjusted.

The entire payment has already been adjusted. When a payment is adjusted, the payment allocation information is updated and a negative payment record is added to the client ledger file. To verify that the payment has already been adjusted, print a Client Ledger Report to see if there is a negative payment entry for this payment. The date on the record will be the date the payment was adjusted.
When a payment is adjusted, a negative payment record is added to the client ledger file. This negative payment record cannot be adjusted.
The payment was allocated when you were running Version 3.4 of Tabs3. Since Version 3.4 did not save the payment allocation information, the payment cannot be adjusted. To reverse a Version 3.4 allocated payment, you must final bill and update credit transactions.