Even though they’re vacationing in Cabo San Lucas, Mexico, the spark between Scott Disick and Sofia Richie may be burning out.
“Scott and Sofia’s relationship is starting to fizzle,” a source tells WHO. “They’re starting to argue more, and it’s bothering Sofia.”
“She thought he would be more serious about their relationship and is finding herself to be more frustrated with him as time goes on,” the source continues. “Everything everyone has been telling her to look out for, she’s now starting to see for herself. It’s really upsetting for her, but she is trying to work through it.”
Disick, 34, and Richie, 19, were pictured cuddling up aboard a yacht in the south of France during the Cannes Film Festival in May. At the time, Richie — who previously dated Justin Bieber — adamantly denied anything romantic was going on, tweeting that the two were “just homies.”
But in mid-September, a source told PEOPLE they had become “inseparable,” and they confirmed their romance via social media later that month, packing on the PDA during a trip to Mexico.
An insider previously told PEOPLE that Richie is “much more invested in the relationship” than Disick, who shares three kids with his ex Kourtney Kardashian, 38.
On the other hand, the source says Kardashian’s relationship with her 24-year-old boyfriend Younes Bendjima is “definitely getting more serious.”
“Kourtney loves the attention Younes gives her, and she thinks he is super attractive. They definitely have an intense physical connection. She loves his swagger and his intensity with her,” the source adds.
Last week, another source also told PEOPLE that things between the couple — who have been dating since early May — are heating up, and that he’s even invited to mom Kris Jenner‘s annual holiday party.
“He makes her very happy,” the source continued. “He is spending more time with her family and everyone likes him.”
The source said that although Bendjima hasn’t met Kardashian’s three kids — Mason, 7, Penelope, 5, and Reign, 2 — “it seems like it’s just a matter of time before Kourtney is ready to introduce him.”
This article originally appeared on PEOPLE