Thanks for the tip on seatcounter. Unfortunately, it looks like this isn't simply a "Frontier hasn't scheduled that far yet" issue. There are seats available on the flight I need in subsequent days.

How good do think my chances are of Frontier adding seats for a Sunday evening flight from Denver to Wichita if I wait?

The Frontier seatmap shows that, of 74 seats, 56 are available, another 8 are available but "premium", and 10 are unavailable.

Seatcounter, however, says there are 10 classes with 9 seats each, and another 6 classes with no availability. That obviously cannot be right (unless they're parking people on the wing or in baggage.) How does seatcounter work?