It's questionable whether it really started the year before the Olympics. I think there's more at play here than mediocre training.
She admitted to lower motivation after winning the world silver, which led to her having a late start to her 2016-2017 season. But Ashley usually fires on all cylinders mid-season and elevates herself around Dec-Feb. The difference was, she went through a major breakup in the middle of the 2017 season. Her long-time boyfriend broke up with her, and her world dropped out from under her before worlds. Combine that with the late start and lower motivation, pressure of 3 spots for Olys, and you can see why her season fizzled out.
From there, it looked like she struggled with injury this season (as a former skater, looking at her crossovers and body alignment this season, I think she was battling subtle injuries), and all kinds of factors including sponsor obligations. There's been a lot against Ashley Wagner in the past 2 years. That said, I do feel that the way she was training around Nationals and her trajectory, that she would have had it together by the Olys/Worlds. It's a shame the USFSA took that chance away from her.
We also have to consider burnout. There hasn't been another skater of her level who has skated as many consecutive seasons at the senior level without a break. No one else is facing that, they're all too young. Caro/Mao/Yuna, they all took seasons or part-seasons away. Kwan took time off in her later years. Ash deserves major props for never doing that, that's gotta be tough.