Help with a question

I recently started to play this game and I just can’t figure out how to upgrade or add more L or X gates/terminals to my airport. I have tried everything but can’t seem to figure it out. I’m in level 15 in the San Diego airport. The gates are all able to go s and m planes but there’s no option to add L or X even though I see them in the gates when I try to buy/upgrade. The only option is to add more Ms. That’s it.
I have large and x large airplanes but only 4 gates to accommodate the flights that are coming in.
Can someone let me know how to upgrade my gates somehow ?

Unfortunately you cannot do that, the accepted categories of a certain tarmac is fixed in the game. In other words, in SAN you can only have 4 tarmacs for L/X passenger jets, 2 with jet bridge and 2 remote.

You can only have 2 additional L/X tarmacs for cargo flights. And that’s plenty of it.

Makes sense now. Thank you so much for the help!