Actually, you were not necessarily raking in profits at start up, it just seemed like you were. The illusion of profit happens when your planes are in their 4 month prepaid leasing period, so it seems like you are making a decent amount of money until the lease payments kick in.
I dont' have visibility into the game, so can someone tell me the OP's fleet and main base?
For most plane types, the lower lease payments of older aircraft are virtually washed out with the C check expesnse, so you'll pretty much make the same amount of money with a new leased 757 as you will a 20 year old, as the difference in lease rate drops straight to the C check (not so much for the russian planes though). The key during the startup years is to continuously roll your profits into new leases and get to a critical mass of planes (i.e. 20+) that are self sufficient - you WILL bankrupt if you try to maintain a 2-3 plane fleet due to the fixed costs within the game's financial systems no matter how good you are.
Assuming we are dealing with a critical mass of planes in the OP's airline, the profiability issues are probably not C check related but either cost related (commonality gone wild), revenue related (inefficient scheduling) or poor plane selection...
In modern time 5 I had a fleet of 737 200 avarage age 15 years. I was attracted by the capacity and relatively low price for leases at the beginning bust up in income but I could not survive the c check too expensive! Done this error different times with always the same result. Second if you fly longest distance fuel draining.
This time I realized that patiance is the bet way I started with only 2 used a/c but not old and place order for new Boeing 737 400 yeas the wait is long so far I have only 15 planes fleet but when c check comes I still make huge profit!! So I can keep order more and more aircraft also, I now moved on 737ng yeah u growing slowly compare to Oder airline but at least you stady and so far so good!!
732's in the MT scenario are usually not the best idea due to their relatively high fuel burn for number of passengers carried (and overall other costs). So unless you're running all of your routes uncontested, you'll probably never do more than tread water with those in MT (and you'll sink if you're flying them on routes longer than 1000nm).