I'm PM about to book a ticket from LGA to PBI for the holiday (well outside of the peak travel days). On all the direct flights on Northwest (K class) it's says that I'm "not eligible" for an upgrade even though, in both directions, the aircraft is an A319/320 that has a first class cabin that is wide open.

Same is true on "some" connecting flights on Delta metal. E.g., on a route from LGA-ATL-PBI, I'm "request only" on the first leg and then "not eligible" on the second, even though it's an MD-88 with an open first class cabin.

I've been flying a lot more with Continental recently. Did I miss something?