It’s a hard knock life for tracking cookies these days.

Ad serving firm Flashtalking analyzed 20 advertisers worldwide throughout Q4 2017 and found that 64% of their tracking cookies were either blocked or deleted by web browsers. According to the research, rejection rates on mobile devices were particularly high—75% of mobile cookies were rejected, compared with 41% on desktop.

Tracking cookies have been a staple of digital advertising since its inception, and the de facto method that many marketers rely on to track and target users online. But the cookie is beginning to crumble as users shift their media consumption to mobile devices, and as browsers and regulators crack down on digital privacy.

Just a few years ago, people spent most of their time online on desktops and laptops. But now US adults spend 1 hour, 14 minutes more on their mobile devices each day than on their computers, eMarketer estimates.

Cookies were originally designed to track users across the web on browsers. It’s more difficult to track a user who spends their internet time opening and closing mobile apps, since those apps operate independently of each other. In the US, users spend 87% of their total mobile internet minutes in-app, according to comScore. This makes it more difficult to use cookies to track people on mobile than on PCs.