It’s only week 3, but the drama has already begun on Dancing with the Stars.

After Tom Bergeron opened guilty pleasures night by dedicating the show to those affected by the Las Vegas shooting, cohost Erin Andrews informed the audience that pro dancer Maks Chmerkovskiy wouldn’t be taking the stage Monday.

“Maks is dealing with a personal issue so Alan is stepping in and dancing with Vanessa tonight,” she said, referring to pro Alan Bersten, who got eliminated last week with partner Debbie Gibson.

However, sources confirm to PEOPLE that Chmerkovskiy did not perform by choice because of ongoing conflict with Lachey.

“There’s a big chemistry issue,” the insider says. “They both have big personalities and that’s not always the best recipe for a good partnership.”

While the former Total Request Live host, 36, has called the Ukrainian dancer the “best coach,” the source adds, “They’ve been fighting a lot.” (Reps for Chmerkovskiy and Lachey could not immediately be reached for comment.)

Pros have been known to sit out of live shows for illness reasons, but not for “personal issues,” one insider said.

This isn’t the first time Chmerkovskiy, 37, has butted heads with his DWTS celebrity counterparts. In 2015, he told The Real of his season 13 partner, soccer star Hope Solo, “That’s literally the only person that I could dislike from my past, present or future.”

On the other hand, Chmerkovskiy and Olympic skater Meryl Davis, who won season 18, brought out the best in each other.

“I feel that I can really be myself with Maks,” she told PEOPLE in 2014. “He pushes me to stand up for myself, whether it’s in real life or on the dance floor. It’s cool.”

He added at the time, “She lets me be creative. I’ve already done more on this season than I’ve done on Dancing with the Stars in 13 seasons.”

Andrews, now 39, has also raved about her experience working with the new dad on season 10. ““The reason why Maks and I worked out so well was because I come from a world where all I worked with was men, and pretty much I’m like a guy in a lot of ways,” the ESPN personality told PEOPLE in 2015.

When it comes to Lachey, though, the insider says last week could’ve been her last performance with Chmerkovskiy: “I wouldn’t be surprised if Maks doesn’t return as her partner for the rest of the season.”

However, a DWTS rep says Chmerkovskiy will compete next week.

Dancing with the Stars airs Mondays at 8 p.m. ET on ABC.

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