Webservices - Another user modified the record

I am trying to update a Purchase order via webservices (published Purchase Order page).

I read the PO, “ReOpen the PO” by extending web-service to reopen, read the lines in PO and modify several lines .

The problem is the last bit, when I update the PO calling Service.Update() gives me the error - “Another user modified the Purchase order …”

What am I doing wrong?

There are code written on the PO lines to update certain field when you modify them, that might be causing you to call update.

As web service does not work as forms or pages in NAV, check for validation fields.

The problem seems to be that if navision throws an error then I (and everyone else) lose access to the webservice until I “update the SERVICE” manually, or remove and re-apply the reference.