PO Dispatch

We use POXMLP process to dispatch purchase orders.  Have the process scheduled to run every 30 minutes.  Had an instance yesterday where the process ran and failed to take the PO's from Approved to Dispatched for a few times.  This resulted in the EDX purchase orders to create multiple lines on the PO_HDR_OUT_EC table and then sending the purchase orders to be fulfilled multiple times.  Any ideas on what would cause this behavior?

Thanks,

Lynn