Please check the below points.
1. Check if any POD concept is implemented
2. check the net value in the sales order
3. Check the delivery document also if there was any incomplete document was pending to be completed.
Some times what happens In in completion procedure instead of putting error message if you put warning message system will allow that particular document to save, but it stops the sub subsequent document,
After all still if you are facing the same issue, please all check the SAP note 374945
thanks,
Srinu.