Are you flying routes with or without competition? When you're starting up, especially in the first 10 planes, you'll want to avoid as much competition as possible.
Also, DOTM is a much lower difficulty level than MT is, simply due to the costs of the era. In a lot of cases, it is virtually impossible to bankrupt a mediocrely run airline in DOTM, whereas in MT, they're sent into a spiral of death rather quickly.
Have you read all the guides about do's and don'ts, startups, route timing, etc?