I think at times the seatmaps can be kind of buggy/balky with DL's IT. Schedule change Saturdays in particular, as FlyerWx notes.

I had a four-segment, three part trip have the seatmaps all lock up today over a 5 minute schedule change. All segments were 'view only' including one 12 days after the segment with the 5 min change, starting late Sat evening. Odd.
I ended up calling the Plat desk this a.m. on an unrelated RUC request and then did an 'oh by the way' and after she looked at my impacted ITN, and a couple holds, she said she had talked with Online and they were debugging based on the issue I had the misfortune of discovering.