Ultimately I have a sublist column on a Sales Order, and I want to trigger a discount WF only if the value in the column = PROMOTIONAL for all lines.

Do you happen to use Withholding Taxes? When withholding taxes are applied it creates a separate line(which may have a blank value for the column you're checking) when the record is saved even though the tax code is entered at the same line as the items. Just an idea.

can it be a Scheduled Workflow? You could then create the criteria in the search format and t=only this SOs will then enter on the schedule?