The string of pop flops is the latest evidence of how difficult it has become for the music business to generate the kinds of genuine moments that turbocharge sales and move the cultural needle, music executives say. Making pop hits has always been a crapshoot. But today, with the world awash in content, TikTok rewriting labels’ playbooks and listeners burrowing deeper into their own personalized niches, even avid pop fans don’t recognize what’s in the Billboard top 10.