Toby Byrne, who has supervised advertising sales for TV’s Fox broadcast network, Fox Sports and, more recently, the 21st Century Fox general entertainment cable networks, stepped down unexpectedly Monday, another sign of the increasing pressure being put on the position across the TV-industry landscape.

The executive left after he and Fox Networks could not come to terms on a new contract, according to two people familiar with the situation. The company said in a statement that it would immediately begin a search for Byrne’s successor, while Randy Freer, president and chief operating officer of the unit, would supervise ad sales on an interim basis. Byrne’s responsibilities will be handled by Bruce Lefkowitz, executive vice president of ad sales; Danielle Maged, executive vice president of global partnerships, and Joe Marchese, president of advance adverting products. It could not be immediately learned if any of the trio of executives was being considered for Byrne’s job.

Byrne’s departure takes place in the midst of Fox’s efforts to sell ads for Super Bowl XLI; just as the Fox broadcast TV season gets underway; and after Byrne’s team has worked to assimilate the National Geographic suite of channels under the unit’s umbrella. In a statement, Byrne said he looked forward to continuing a relationship with the company and was “excited to explore the next chapter of my career.”

The shift in Fox’s ad sales ranks is the latest to take place in an industry where the sale of advertising – once a turnkey process – has become exponentially more complex. As more viewers turn to mobile devices and streaming video, and traditional viewership erodes, getting Madison Avenue giants like Procter & Gamble and McDonald’s to spend the same millions on TV ads has become a more daunting task. Advertisers have placed more pressure on TV networks to come up with methods that help them target more discrete audiences and align commercials with the programs most likely to reach them.

In recent months, Fox has burnished its development of technology that asks users of new TV-viewing technology, like video on demand or streaming video, to interact with a particular commercial message in exchange for seeing fewer ads overall. Whether that method is enough to keep millions of dollars coming in remains to be seen. Under Byrne, Fox has also proven willing to take big creative swings: last season, the company arranged to have Pepsi be utilized in a three-episode arc of its hit drama “Empire,” culminating in a commercial during the show that tied in to the story.

Other TV companies have also grappled with shifts at the top of their sales organizations. Indeed, last week both AMC Networks and NBC Sports announced that two longtime senior sales executives would step back from day-to-day responsibilities. At AMC, Arlene Manos, who helped the company convert several commercial-free cable networks into ad-supported outlets, said she would focus in the near future on bigger projects, while NBC Sports said Seth Winter, who has supervised the process behind some of NBCU’s biggest properties – “Sunday Night Football” and the Olympics – would move to a consulting role. At Discovery Communications, Joe Abruzzese, the company’s longtime head of ad sales, is expected to step down later this year. Jeff Lucas, the former ad sales chief at Viacom, recently left that company to take a similar role at instant-messaging outlet Snapchat.

Executives said Monday they were cheered by Byrne’s recent work with the TV “upfront,” during which TV networks try to sell the bulk of their ad inventory for the coming season. Fox Networks saw volume gains of about  5%, while the Fox broadcast network secured between $1.47 billion and $1.64 billion for its primetime schedule, according to Variety estimates, compared with between $1.43 billion and $1.56 billion for the 2015-2016 TV season.

Byrne rose through the ad-sales ranks at Fox after joining the company in 1996, having served previously as a media buyer at Omnicom Group’s BBDO. He came to more prominence at the start of this decade when he was named to succeed Jon Nesvig as ad-sales chief for Fox Broadcasting. Nesvig cut a legendary figure in the industry, serving as a kind of adult in the room as younger programmers worked to turn Rupert Murdoch’s late-1980’s vision of launching a fourth broadcast network to compete with ABC, NBC and CBS. In some ways, Byrne’s challenge was even more daunting than Nesvig’s: He has had to refashion an ad sales department in which executives were focused on selling, say, broadcast or cable inventory into one in which everyone was prepared to put together larger packages that included digital inventory.

Fox will have to press on in his absence. The company has been working to sell the bulk of its Super Bowl ad inventory at the rate of more than $5 million for a 30-second spot.