It's a puzzling thing, and my impression is that no-one has really been able to explain this. I've been very distant from football for a long time, but it's seemingly lasted through many managers, many signings in every position imaginable, and through different formations. I was young at the time, but I remember a big deal being made of Veron not living up to his potential at United, yet that sort of thing seems completely commonplace nowadays. There strikes me as being a trend actually where players will join, play well initially and look like really top footballers for a short period, but then tail off quite quickly and start to perhaps lack confidence in their own expressiveness on the pitch. Kagawa was like this, and even Van Persie went through a massive dry spell in his first season. People were really praising Matic when he first arrived, but now... not so much. It's like there is an established pattern of behaviour that players sink into shortly after they join, regardless of position, manager or formation - like pouring a teaspoon of boiling water into a glass of water that's just been in the fridge...
There's a lot of talk of getting the right formation, and playing players in the correct position, but there comes a point I suppose when you have to ask why all of this should require such a level of perfection. The classic example to discuss is the 2-0 game against Arsenal where United fielded about 7 or 8 defenders, and another is when United went through the latter half of the 05/06 season playing great football with O'Shea and Giggs in the centre of the field. Then it got to about 11/12, and this sort of thing stopped happening, which possibly contributed to the circumstances of Pogba leaving given that it was no longer the case that United could throw 11 players onto the pitch regardless of formation and natural position and be confident of getting a result.
Not putting forward any answer to all this, but I've been thinking about it for years, and at the very least it's an interesting situation.