How exactly are you updating the sales documents? Are you updating them directly from VA02 transaction or through some program? If you are updating the document from program using BAPI, then you will have to make sure that BAPI transaction Commit or rollback is done after BAPI call.
If you are updating sales document manually by going into standard transaction VA02, then follow below steps.
Find out the lock object name name for the sales order. I think you will get it in SMCL or SM12 tranasction. Once you got the lock object name, note down the Enque and deque FMs.
Then go to VA02, enter sales order number, turn on debugging mode, hit execute/enter to go into the document. This will open debugging mode where you can set a breakpoint at Enque, Deque FMs.
Once document editing is finished & document is saved, program should deque all the tables under same lock object. Check if Deque FM is getting called or not.