I'm trying to add 'Preferred Stock Level' from the Item record to the line items on a Sales Order I've created a Transaction Line Field, with Applies To set to Sale Item, Source List set to Item, and Source From set to Preferred Stock Level, but each line item on the Sales Order shows nothing, even when that Item has a preferred stock level - am I missing something?

Are you looking at orders already created or creating a new one and not seeing it pop up? You might have a sourcing issue, where transactions already in the system are not going and actively sourcing information unless they are modified/edited.