Just call DL. I had a great Chisholm agent last night who spent about 30 minutes exploring all of the Alaska availability for me over a 6 week period until we found seats (which we did). She told me that they can only offer Alaska seats at low mileage. I managed 2-F transcon reservations at 45K/each...80K on DL metal w/ connection or 40K in coach. For a quick 10th anniversary trip, it was miles well spent.
Also, about a month ago, I was seeing lots of Alaska availability at delta.com. That isn't the case any longer in my experience. The Chisholm agent said they even have to log into a different system to search/book Alaska flights.