There’s been some success: More people were working in offices than entirely from home in the U.K. for the first time since the pandemic, a recruitment agency found in October.

Yet some employees continue to defy back-to-office mandates, forcing companies to find new ways to keep an eye on who makes their way in and who doesn’t. At accounting giant EY, that’s been by tracking the turnstile entry of its employees, the Financial Times reported Monday.

Anonymized turnstile access data was shared with some partners at EY to show how often people come into the office in the U.K., staffers at the Big Four firm told FT. Sometimes, data on attendance was even correlated to midyear performance reviews, one of the sources said.

The big takeaway? At least 50% of the people in some teams were falling short on meeting the return-to-work policy, currently at least two days a week, a person familiar with the matter told the outlet.

The data illustrates the ongoing struggle for employers with luring people back into the office even as we’re in a post-pandemic world. Hybrid work was hardly the norm prior to lockdowns (although it had been slowly increasing). However, the flexibility and benefits to employee well-being have been significant—and employees want to hold on to them.

The push for more face time

Bosses argue that there are clear upsides to having people in offices, so many of them have been firm about advocating for it. Goldman Sachs CEO David Solomon was early to call working from home an “aberration” in the hybrid-work debate. Other banks, including Citibank, have also said they would monitor employee attendance through their ID card swipes as a proxy for attendance. Earlier this month, Bank of America issued letters (the company calls them “letters of education“) to workers flouting return-to-office rules, warning them of “disciplinary action.”

It’s become apparent that bringing people back into offices is not as simple as pivoting to old times. Return-to-work mandates have their fair share of shortcomings too, given its impact on employee satisfaction and future recruitment prospects.

But returning to the office can be beneficial, especially for employees in early stages of their career, Kevin Ellis, U.K. chairman at fellow Big Four company PwC, said a few weeks ago. His advice for junior workers looking to succeed in their career would be to spend more time in the office.

A PwC spokesperson told Fortune that the firm keeps tabs on aggregate office usage for several reasons.

“It helps us gauge employee preferences and assess whether our hybrid policy is working, so we can plan accordingly,” the spokesperson said, adding that its policy of employees spending 40% to 60% of their time at work with their teams, has been “working effectively.”

A spokesperson for KPMG told Fortune: “Our firm’s hybrid working model balances the flexibility of working from home with the importance of collaborating and learning in our offices or at client sites. There isn’t a one-size-fits-all approach.

“Attendance in our offices has continued to increase year on year, with around 11,500 individuals spending at least one day a week in the office.”

EY declined Fortune’s request for comment.