By Joe Flint 

The whodunit crime dramas that CBS Corp. counts on to drive its broadcast network's prime-time lineup are dealing with an unexpected plot twist this season--declining ratings.

The audience for "Elementary," a modern-day take on Sherlock Holmes, has shrunk by 15% compared with last season, according to Nielsen. In the key demographic groups of adults ages 18 to 49 and 25 to 54, "Elementary" is down 28% and 18%, respectively. Also sliding are "The Mentalist," "Person of Interest," "CSI," and "NCIS," the network's No. 1 drama.

"They have lost some ground," said Edward Atorino, a media analyst with Benchmark Co. "Maybe some of the shows got a little tired."

CBS has been the most-watched network for six-straight years and its formula for success is primarily tried-and-true crime dramas that have broad appeal. Often, if a show works, such as "NCIS" and "CSI," CBS makes spinoffs of them, as it did this season when it launched "NCIS: New Orleans." The strategy has paid off for CBS in the ratings and created a financial windfall for the company in syndication sales.

But now many of CBS's most reliable performers are past their peak, and it is showing in the Nielsen numbers. It isn't limited to the dramas: A few of the network's comedies, including "Two Broke Girls" and "Mike and Molly," have also seen better days.

CBS executives say they are pleased with the network's performance so far this season. "If you are handicapping the four networks going into the second half of the season, there is just no contest," said CBS Corp. Chief Research Officer David Poltrack.

The conundrum for CBS is that the shows in question still deliver relatively strong numbers by industry standards, but are no longer good bets for the long-term. The scenario CBS wants to avoid is the one NBC faced a decade ago when it had to pay more and more to keep aging hits such as "Friends" and "ER" on the air because it was unable to develop successors.

"CBS has been in that glass half-empty, half-full scenario," said Sam Armando, a senior vice president at the media-buying firm Starcom. "You can't cancel these shows because they are still strong, but you know going in they are not going to lift your network anymore."

Despite the challenges, CBS remains in first place this TV season, with an average prime-time audience of 11.2 million viewers, up 1% compared with last season and still far in front of rivals, according to Nielsen.

In the 18-to-49 and 25-to-54 demographics, the network is down just 9% and 6%, respectively, not including Thursday night NFL football telecasts that would boost the numbers.

CBS's issues look like luxury problems in comparison to some competitors. Take its Sunday night football games out, and Comcast Corp.'s NBC is down 3% in total viewers and 14% in adults 18 to 49, according to Nielsen. 21st Century Fox's Fox network is off 16% in viewers and 15% in adults 18 to 49. Thanks to new successes "How to Get Away with Murder" and "Blackish," Walt Disney Co.'s ABC is up in total viewers and flat in the key demographic segments.

Like all the major networks, CBS is contending with industrywide challenges such as audience fragmentation and the onset of cable "cord-cutting."

Bright spots for CBS include new dramas "Scorpion," about tech-savvy crime stoppers, and the "NCIS" spinoff set in New Orleans. Two veteran shows--"Criminal Minds" and "The Good Wife"--have improved their ratings this season.

CBS Corp.'s Mr. Poltrack played down the decline in ratings for many of the network's shows. "CSI," he noted, has aired 367 episodes and still gets more than 10 million viewers in a competitive Sunday time slot. As for "Person of Interest" and "Elementary," Mr. Poltrack attributed their drops to tougher competition than in previous years.

Media buyers aren't ringing alarm bells yet. "Within a highly fragmented marketplace, CBS remains beachfront property," said Andrew Donchin, chief information officer of Dentsu Aegis US.

CBS has long portrayed its stable schedule as a selling point to advertisers, but significant changes likely will be in store for next season. CBS has already said this will be the last season of "Two and a Half Men" and "The Mentalist."

"What this signals is CBS will do a bit more refreshing than they typically would with next year's schedule," said Irwin Gotlieb, chairman of media buyer Group M, a unit of WPP PLC.

Write to Joe Flint at joe.flint@wsj.com

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