Lately (at least for DL)the system will not display all rows of DL F seat maps even if seats are available in that row.
Eample DL 1600 ATL-BOS Dec 16 it will not display row 6 even though row 6 is empty and has 4 open seats.
If you use the graphical option (that goes to Delta.com)it says the seat map is not available and to call DL.
But if you go to Delta.com and select the flight the seat map is readily available.