I’ve had the same experience. Get a fare, try to book the fare and whoops that fare isn’t available, how’d you like to pay a couple hundred bucks more.
Since we only have US Air or Delta, and it costs twice as much to fly out of the Twilight Zone Multi-National Airport and Subway Shoppe, I look at several sites and pick the best. Saved a ton on are last trip to the Land o’ Cheese, Sausage and Beer with Expedia.