Header Modification ("Affect detail lines") from workflow



I'd like to update the shipment site of a sales order and have it affect all of the lines as well, when a workflow condition is met:

For example, I have a workflow that will use the AWRKUPDFLD to update the STOFCY field of a SOQ line, but it only does the first line and not all of them. Contrary to this, if I use the action to update the SOH STOFCY field instead, it does not affect any lines at all, only the header. 

What do you think is the best way around this?