I get the general Merchant advancement rules. I'm just finding it odd that none of my Merchants ever seem to advance on this one specific resource.

In my last game, I had a Portuguese Merchant sitting there for twenty plus turns with no improvement while his colleagues trading other, lesser resources closer to home improved significantly. I've had the same problem with Scottish, English and French Merchants going there to bury their careers. That said, In the campaign I just started, my Danish Merchant improved after about five turns, so I guess it isn't bugged.