I am having same problem on a flight tomorrow from DL 1929 SEA-ATL. I have my seat assignment but when I click on the seat map on my itinerary a pop up says:

"Sorry , you cannot select seats for this flight at this time. Please check back later or receive your seat assignments at the gate prior to boarding."

It's been that way for days now. It shows my seat assignment but no other seats available. Can this be a flight that is truly overbooked or is a change of aircraft on the agenda? I would think that would have happened by now since it's little over 24 hours from now.