I've found that if I manually search for the delta flight number, Cortana brings up the status info with a ":Track this Flight" option underneath.
Not ideal of course.
I also think the flight needs to be withing 28-30 days. I just tried to add some future flights occurring in late April. I manually put in the flight number and it gave me the status for the flight today. I clicked on the track this flight option, then opened my Nnotebook -> Travel. I clicked on the flight number, and there was a drop down to pick the day of the flight. It only went 28 days into the future, so I couldn't change my own flight yet.