A weight loss 𝐩𝐥𝐚𝐭𝐞𝐚𝐮 is when your weight, your body measurements, and your body fat have not changed for a month or more. Any less time is a 𝐬𝐭𝐚𝐥𝐥.
There is no pat answer to the question of why a person who claims to be “doing everything right” stops losing. Every person has his or her own stumbling blocks, including health and medication issues or, and this is most often the issue, drifting off their diet, many times without realizing it but, in my experience, most often it’s because they choose not to acknowledge it!