Oops! Apparently, no one is rushing to the box office for a piece of Britney Spears. She’s been struggling to sell tickets to her “Piece of Me” Las Vegas shows since she started her residency at Planet Hollywood in 2013, and plenty of seats are regularly available.

One source attributes Spears’ sagging box office to the return of Vegas powerhouse Céline Dion, who made a triumphant comeback to the Strip in August.

“It’s a Vegas war now that Céline is back, and Britney is losing in a major way. Céline has sold out almost every show,” said the source.

However, other insiders have a different explanation. “Britney got off to a really good start, but her fans aren’t particularly Vegas people,” said one source. “It’s hard to keep people interested in an act like Britney Spears since she’s a 1990s, 2000s pop star. That fan base doesn’t really come to Vegas to see shows.” These days, “They’re there for the EDM scene and DJs.”

Another insider told Page Six, “[Spears’ ticket sales] have nothing to do with Céline. Acts like Céline and Elton John have had adult fans for many years and they go to Vegas to see them regularly. Britney’s fans are young adults who grew up with her as little girls. How many times do they [go to] Vegas?”

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The source added of Spears: “She can fill arenas, but a residency is another beast. A residency is different from a worldwide tour. Those seats have to be filled every night, and that’s difficult to do with a younger fan base.”

Spears began with a reported deal for $475,000 per show. Other acts to have hit Sin City with residencies: Mariah Carey and Shania Twain. We hear more country acts will be joining the group. In the meantime, Spears still has time to boost her sales.

A rep said: “Caesars Entertainment and [Spears] have a great relationship built on the massive success of her ‘Britney: Piece of Me’ show. So muc​h so that they recently signed a two-year extension based on the tremendous desire from audiences to see the show from ​around the world.”

This story first appeared in the NYPost.