Not Receiving Tracking Numbers After Shipment

Hi, I'm posting orders/shipments into ship rush web via the custom post-in method ShipTransaction (shipment + order) xml document to your web service (https://api.my.shiprush.com/Integrat....svc/add/order). We can import the orders fine and learned that printing in batches of 10 or less work best. We have a callback url set which ship rush web posts the updated shipment xml to. We have noticed recently that 25% of the shipment xml received is missing the tracking info from the //Request/ShipTransaction/Shipment/Package node. Does anyone know why this happens and how we can prevent something like this from happening (race condition)? The labels print with the tracking number so we know we are able to reach the carrier's service as well. If there isn't a proper fix/explanation for this, is there a way to manually resend the shipment/tracking info to our callback url?

Thanks.