Instead of starting a new thread I searched and found this, so hopefully I can get an answer to my Question. My girlfriend is flying PHL-IAH-MID (Jul 8-Jul22), she was able to select her seats for all flights except IAH-MID (Jul 8), according to the CO website all coach seats were filled and she didn't have the option to select a seat.

But availability shows:
Cabin class: C9 D9 Z9 Y9 H9 K9 N9 B0 O9 V0 U9 Q0 I9 S0 W9 T0
Meal:
Distance: 717 mi
Stops: non-stop
Flight Time: 2:10
Boeing 737-300

Since this is the last flight of the night to MID, she can't miss the flight or she'll be waiting until the next day. Is there any reason no seats are shown as open on CO's website?