According to The Hollywood Reporter, staffers clashed with Kournikova, 30, while some contestants found her unsympathetic.
But her rep tells PEOPLE that "Anna was 100 percent not fired from the show."
Kournikova issued a statement saying, "I enjoyed my time on the Biggest Loser ranch. Although I will not be returning as a full time trainer on season 13, I will always be a part of The Biggest Loser family and my commitment to bettering lives through health and fitness will continue."
"The show just wasn't the right fit for her," a source close to Kournikova adds.
While Joe Mitchell, who was sent home from the ranch last week, says that Kournikova fixed him, other contestants felt she had additional issues to contend with.
"One of the things about The Biggest Loser is that there's a difference between someone being able to train people versus training overweight people," former contestant Patrick House, who enjoyed his time working with Kournikova, tells PEOPLE. "Bob and Jillian are great at training obese people and pushing them to their limits and getting more and more. I think you have to adapt to that and I think that's what she struggled with."
The former tennis pro certainly had big shoes to fill after Jillian Michaels' departure at the end of season 11.
"I think one thing Anna did was that she never tried to be Jillian," Biggest Loser alum Hannah Curlee told PEOPLE. "For her that was her biggest achievement. Because no one else is going to be Jillian, no one is going to replace her."
Fellow contestant Olivia Ward agrees. "Anna came in and tried to carve out her own niche," she explains. "I have to give her kudos for that."
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Anna Kounikova
