Hi, I'm a ELVIS the asset creator of the ICN intl airport.
I heard lots of complain of plane doesn't landing.
I don't know the reason exactly. But here is some ways to solve the problem

1.Restart game
Somethimes, restart can be solve the problem

2.build more taxiway connection
If you connect airport wih 1~2 ways, it cannot be work.
Then, try to use more connection to main airport.

3.Use Unlimited outside connection
If you don't use this mod. It cannot be use airport.
Subscibe it and vitalization it.

4.Connect road to first floor
Bug was reportted about Not much visitor
The reason was player use only connect second floor road

5.Draw New Flight route
If you didn't solve the problem
try to connect runway and flight route agreement