If the invoices have been posted already, why are you entering cash as a UP (unapplied) detail? The invoice is in the data and ready to be applied to.

When entering a cash transaction for a customer, you should see a list of all of the outstanding invoices for that customer in a grid at the bottom of the screen - if the Cash Entry method is set to 'Normal'. If this is the case for you, all you have to do is enter the amount of the check into the 'Payment' field, and use the space bar to apply cash to the selected invoice in the bottom grid. A 'UP' transaction is not required, and really should be used only when the cash precedes the invoice (such as the case for a deposit, for example).

If your Cash Entry method is 'Direct', you will not see the list of outstanding invoices in the grid, but rather a list of invoices you have selected to pay. When you click New to add an invoice to the list, you can type the invoice number into the Document Number field (or select it from the finder). This will load the job number and the amount outstanding from the invoice on file.

It may be that if you post a UP cash detail with the same document number as an existing invoice, information doesn't get matched up in regards to JobCost Payment information. But this isn't the correct way to perform payments to existing invoices in AR.