One bad transfer window can have that happen. Which was the case in 2010 and 2017.
The 2010 summer transfer window (Ramires, Torres, Benayoun, and Luiz) set us back tremendously, in spite a couple of those players having big moments for us. We never adequately replaced Deco, Ballack, Joe Cole, and Carvalho in the slightest, which was why we had to wait so many years to actually compete for the title.
We are now seeing us finally being able to compete after the 2017 disaster window.