It’s no secret that Chipotle has struggled to earn back consumers’ trust. Following a series of highly publicized food safety scares—which saw dozens of people sickened by E. coli across the U.S.—the beleaguered burrito chain has tried everything to get back on top.

Now, despite millions of dollars in food giveaways, and a number of new safety precautions, Chipotle’s co-CEO has hinted at just how far the company still has to climb.

According to Fortune, Steve Ells—who also founded Chipotle in 1993, and serves as the company’s chairman—said that only half of the chain’s stores get an “A” or “B” grade. The company’s other 1,000 locations, specifically those located in metropolises like New York, Philadelphia, and Boston, only deserve a “C” grade by Ells’ estimation.

“I’m not satisfied with the rate of recovery and the quality of the restaurant experience,” Ells said at a conference with Barclays on Tuesday. “We are not offering the necessary guest experience.”

Ells attributed those C grades in areas like New York City to “dirty dining rooms, messy soda filling stations and slow lines,” according to Fortune. As Chipotle has struggled to rapidly train its employees on new health and safety standards, the company has seen a higher turnover of workers, and believes it may have let customer service fall by the wayside.

“We threw a lot at our employees,” Ells explained. “We took our eye off the ball on customer service.”

Shares in Chipotle’s stock apparently “plummeted” on Tuesday after the executive revealed he was feeling skittish regarding the company’s 2017 profit forecast. Earlier this year, reports signaled that the company's investors were looking for Ells to step down as chairman of the board

[via Fortune]