Robert,
It may be my experience, you have to go and manually check the additional conditions assigned to the respective product as Okay otherwise system will not let the product be preferentially eligible.
Once you load the preference rules depending the rule of origin you will have the additional conditions.
you can use the below tcode or follow the path to access the additional conditions check for the material.
/n/sapsll/pref_prprfsc
or Go to Risk Management section in the GTS Menu Legal
click on preference processing Master Data
you Will see Assign conditions to a product on your right hand side of the screen.
Give the inputs as needed and you can validate the additional conditions for the given product and HS combination. Thanks.
Regards
Raj