EDIT: I finally figured it out just after posting this, and can't find a delete button. Sorry for the mess.
My 20 other routes make sense and are doing fine, but this one is capped at 66% load factor, and I don't understand why.
Economy class
99 demand
100 seats
66 pax
Route reputation 42 (no lower than my other flights)
Deafult price
No competition
Lowering the ticket price below default doesn't change the load factor at all; increasing it will lower it from 66%. The only difference I can see is that this is my only route longer than 1,000mi, but I don't see anything in the game that makes that relevant. Am I missing something?