Motivation is obviously a factor, but also fitness and injuries are in play here.
Raphinha and Pedri are two of the most important 3 players in our system and both got multiple injuries.
While Kounde is playing like he has aged 5 years, right after being one of the most played players in the world, Lewandowski is no more dependable starter as he aged even more, Lamine suffered chronic injuries as he has been overplayed for a player of his age.
Lack of reliable depth is killing the intensity as the team is exhausted, and an exhausted team can't press, and if you can't press while playing attacking footballer, you can't defend
I don't think it's as much full belly syndrome, but normal erosion, both physically and mentally due to 3 cumulated factors.
First is the squad depth which has been done to death and discussed numerous times.
2nd is the injuries this season, not only frequent and targeting the best players, but longer in duration (and I also consider the first season as part of the reason since it all adds up), affecting multiple tricky games.
3rd is on Flick himself. He is just anti-pragmatic. And pragmatism is a quality.
His system has worked well right from the start with these players. They knew all along what he wants and how to deliver that vs some of the best sides early on. It took almost no prep work.
And if this is the case, then maintaining freshness is his main priority, since this is foundational for his football. Fans will just need to understand that playing super-energetic football vs some random La Liga side is not the most important priority when the squad is short and your ultimate goal is to win titles in May and June.
But Flick just plays the same. I have seen us pressing the keeper whether it is a CL semifinal, a Clasico final, or a game in Copa vs some Segunda side. This is very taxing on the body when you do it almost every 3 days with largely the same core. There is just no pragmatic and strategic reason to do this. We run too much, too hard, too long to secure points vs teams we could have beaten easily on a more conservative, energy saving mode.
He has made no compromise at all, and he is in a position where what he does will collapse at some point, and in a noisy way. I have seen numerous times that he believes in focusing on the next game only, but that's not something I agree with. You are far better served building up the pace and conditioning the side to be in the best shape in the 2nd part of the season.