This is something I posted about a month ago elsewhere about Chelsea's problems.
Long and short, this comes down to the years of mismanagement by the board. While they have gotten the swapping coaches in and out right, more often than not, the communication between board and coach has failed, on plenty of occasions. And this goes all the way back to when Roman first bought the club. The board have given out so many mix messages over the years of what they want, that they have become a parody of themselves. Must be self sufficient, want to develop players from our academy, wants winning football, but needs to be with style. Still find ways to win and finally gets the style, but abandoned it within 18months. Wants fresh approach with youngsters being developed,doesn't click with the old guards, discarded. And then the rinse of repeat, of buying players the coaches do not want or need. Signing players either for or to tempt Sven. Signing Sheva and Ballack when Mourinho wanted other players in and then the next summer, no money to spend like they had done, but on Bosmans or cheap transfers.
Then the Ancelotti era, won the title in spectacular style with an already aging squad that had been together for a while. He wanted to bring in his own players and freshen things up, given limited funds in the summer window, as the squad was already good enough. Gets Torres last min in Winter window when Ancelotti wanted someone entirely different in a different position and the team fails season in both league and Europe.
Brings in AVB, told he has a blank canvas to do whatever and between his arrogance and the board not supporting him, failed. Di Matteo comes in, pulls off the Miracle of Munich, get transfer funds, the squad get overhauled and experience players leave, but the players fail to gel, they get knocked out of the CL before the group stages end and Di Matteo sacked. Back to Mourinho again, comes back, gets backing to win the title, but the slide in the teams form second half of the season is a concern, Mourinho wants more players in, doesnt get those players and the team falls apart.
Brings in Conte, he wins the title, thinks that gives him credit to bring in his players, to change things, but only a 1 or 2 of his players are bought in and he then goes into a running verbal battle with the board in the media. Then not only do they keep Conte in charge of the team when everything seems to be broken down on all levels, he is waiting to be sacked, the club don't sack him, he is still taking pre season 3-4 week before the season starts, along with the new transfer window shutting early rules coming in. Sarri gets half a pre season to implement his philosophy. And now Pulisic gets bought without the current coaches knowledge.
All in all, it is pretty amazing how Chelsea have continued to win, given the circumstances of the bipolar nature of the board. Yes, their way has been successful, even more so then it should of done, but the long term effects are slowly creeping showing themselves now.