Payment Amount is NULL at "Invoices Pending Review" stage

Hi,

1. We submitted a Payment run which selected around 70 invoices. The PPR is in "Invoices Pending Review" stage.

2. User clicked on Start Action button and found that for some of the invoices "Payment Amount" is Null or Blank. User couldn't proceed with any further action (like removing some invoices, adding others etc). For other invoices, everything looked fine

3. We had to manually add the Payment Amount for the missing fields.

Why is it that Payment Amount is blank for some invoices and others are fine ??

Any inputs on resolving this issue is appreciated

Thanks,

Siva