Hi Jocelyn,
Good day! Can you please check under Setup > Import/Export > View CSV Import Status > Message column > If how many record(s) were indeed imported successfully? Under CSV Response column > You may also click on the link to open the Result File and verify any CSV-related error message(s).
Does the custom field that you are trying to update could be found on the Sales Order’s Main field or on the Line Item (Under Items tab)?
If on the Main Field, then what you basically need is the Internal ID of the Sales Order to be updated and the New/Correct Value of the Custom Field. If on the Line Item, then apart from these two fields, you also need to include and map the Item Line ID and keep the default Advanced Options settings as it is (no need to enable the Overwrite Sublists).
Hope this helps. Should you still need further assistance, you may feel free to create an online case so that our CSV – ERP team can further investigate. Thanks!
Regards,
Judy