I have 107 gates with 14 large stands, 93 medium stands, 8 runways with 2 medium parallel departure runways, 2 medium parallel landing runways. No matter what, 95% chose the same runway and I get the dreaded airplane stack of a bunch of planes. 2 large runways - 1 dedicated for landing and 1 dedicated for taking off. Same with another medium set of runways.
There is a setting in the operations menu called distribute planes evenly across all runways.
Tried using both modes, no change, bunches of planes congregating next to only certain runways and leaving two other runways empty and unused. Multiple checked to make sure everything is hooked up properly and should be working but no change.
Try reloading the game after switching, sometimes takes some time to go into affect, like a day or so and a reload or two
Just like real life senario, which does not happen in the game. If an aircraft has to abort takeoff they need to taxi out of the current runway, but not to hold up other aircraft which needs the run way to take off. Therefore, you need to create an exit point or points from your runways and make sure you also choose the correct taxi way arrorws going into and out of both runways. Hope this helps.
Can you share some screenshots of the airport layout?