This was what I was thinking when I read through this thread. Everytime there is a schedule change Delta.com prevents you from seeing your seats (that usually are still assigned). Is there a red "schedule change" on the flight info?

Call Delta. They will read you the new itinerary (which you have already looked at online...) When they unlock it you will have access to changing your seats around. (Unless it is a NWA plane??)

Too bad the Delta.com site doesn't have a "accept changes" click so passengers don't have to call reps everytime a change happens.