Uhhh... maybe you left out too many verbs when booking your ticket

Seriously though, could be a computer error or your name might not be in the PNR exactly as it is in your WP account (that has been rumored to be a problem here on FT.)

Also, just because there are seats available for purchase doesn't mean you'll get an upgrade, there must be seats in the F bucket 3 days prior.

And like magic said, if you booked the ticket as a silver, that's what your record will indicate (it happened to me when I went from silver to gold.) You need to call NW and have them update that ticket.