Partner says they brush twice a day and still gets tar tar build up

[we<3teeth]

They don't drink or smoke. Apparently they brush twice a day, yet they get really bad tar tar build up on their teeth. I know they need to go to dentist to get the tar tar removed. I want them to prevent it happening in the first place. There was a small period when they were younger that their teeth weren't looked after properly. Apart from the tar tar build up, the teeth look fine. Are they lying to me about brushing twice a day?