I spent a while figuring this out - over several FM's. I'm still not certain I have, but I do have some stuff I like to try.
Some helpful soul on the forums pointed out to me that as you go on those incredible runs, other teams start to look at yours differently, so you need to adjust. Not so much on a per-game basis - I actually do my best to limit game to game fiddling. Just - how you go about playing on a month-to-month basis or over the course of seasons.
Not sure I have stellar advice here and I'm not immune to changes in form / overall success over the course of a season, but I did learn to manage my expectations a bit. If you're a lethal attacking team, teams will try to figure out how to stop you or counter you. If you're the best defensive team in the league, teams might decide to keep it tight an challenge you to beat them. Don't just think about how to beat them - think about how they're going to try to beat you.
Probably worth pointing out that not many top division teams go on 40-game unbeaten runs. You're going to regress to the mean. Take a look at your data and see where you are, and what might have changed.
Other possibly helpful hint - I tend to find that experience, determination, leadership are all generally massively underestimated by human players, and not by the computer. A team full of wonderkids doesn't tend to be brilliant at unpicking why things are going wrong.
Good luck. It's a fun challenge...:D