If you are printing picking slips or any other document type in OrderEntry with the 'Use Current Customer specification' setting, it will not load the customer spec into that screen. It is OK that the field is blank - when using the 'Customer spec' option, the spec on screen is only used as a fallback if there is no defined spec on the customer record and the Invoicing tab. Simply print the picking slip, and if there is a spec set on the customer, it will be used.

FYI if you want to assign a default spec when printing, it is done from the File / Workstation Options area, in the same place that default printers are set.