I'm playing in GATOW. Last week my enterprise got a 37% profit margin and this week it's 7%. I found the price of Maintenance is highly increased. Let's raise an exemple, TAO-PEK. 600km route. Last week it earns me 7000AS$ using a brand new A320, now the number is 3000. I didn't change the maintenance service provider, nor made any changes to the scheduling. Now I have to switch to a cheaper one but the margin is still incredibly low. (Very sorry for my friends using Huge 757 and old 767 fleets)
Is there an update or something causing this situation?
You are using 12 different maintenance categories, each category after the third adds an extra 15% to the maintenance cost of your entire fleet. With 12 categories that's an extra 135% in maintenance costs. Meaning you are paying more than twice as much in maintenance as you would if you had less categories of planes.
There's just no logical reason to use both both 737s and A320s as they serve the same function, best to just choose one of these and save the extra costs. Also you have a single 777, two F70s, 3 ERJs, each of these add a new category and raise the maintenance costs of every single airplane in your fleet for very little benefit.