I had a similar issue, seemed kind of random. Only thing I would suggest is when you are subscribing to a custom airline, take notice of the aircraft types (templates) that it uses. usually on the right hand side of the screen will be a listing. If you don't see green check marks next to them, that means you do not have that aircraft template yet. I would suggest subscribing to those aircraft templates first, maybe even exiting the game, then subscribing to the custom airline.

Hi is importent , that you have all aircraft mod , witch the airline mod needs , if you not have that , the mod not works , you see that if you have at Download Missing mods , one or more red Missing Markes in. Then you need to subscribe them. But at moment there are also a problem with subscribing/unsubscribing planes. If you have that , you must close SA , and unsbscribe and subscribe that planes. (somethimes 2-3 times) until they work

Great. Thank you for your help. It works perfectly. Love these mods. Great respect to all modders.