Walmart US CEO Greg Foran said half his store visits make him "grumpy." Reuters

Walmart US CEO Greg Foran said on Wednesday that some of the company's stores are failing to meet his standards.

"I get out to stores every single week," he said. "About half the time I'm ok with it, and the other half I'm grumpy."

Walmart is putting pressure on store managers to improve operations at "these stores that are letting us down," he said.

Walmart US CEO Greg Foran said on Wednesday that some of the company's stores are failing to meet his standards.

"I'd say we're at about 50% of where we should be," Foran said at the UBS Global Consumer and Retail Conference, according to an audio recording of his remarks. "I get out to stores every single week. About half the time I'm ok with it, and the other half I'm grumpy."

He said there are opportunities for improvement in customer service, inventory levels, in-stock levels, and assortment.

"We've sort of made some good progress, but the unvarnished truth is that we have not even gotten close to arriving," he said. "For sure we are doing better, but our peers are still a long way ahead of us."

He also cited the quality of fresh food and private brands.

"Fresh has been a big deal, and we haven't even got close to arriving on fresh," he said. "I still go into stores and the wet wall - leafy greens - quality isn't good … they haven't got their PI [perpetual inventory] right. They're not rotating the merchandise. But it's better than it was."

Read more: Ex-Walmart exec says theft helped kill Walmart's cashierless checkout technology

Foran said Walmart is turning up the pressure on its store managers to improve operations.

"The key to running a store generally comes down to the quality of the store manager, and if we don't have a good store manager, then the person I hold accountable is the market manager," he said. "We're at a point now where we're putting a lot of pressure on that group to step it up and to get these stores that are letting us down to a level which we know they can if we get the right team in place."

Foran also talked about the challenges of implementing change in an organization the size of Walmart.

The company has 4,700 store managers, 10,000 comanagers, 60,000 assistant managers, and 120,000 department managers, he said.

"This is a massive change program," he said. "This isn't going to happen in five minutes, and nor should we expect it."