The face of my character deforms when I rotate the head

I made this character from zero, and I’ve learned a lot making it. While playing with the Shape Keys, I decided to give her teeth so she could smile, but thanks to that I noticed something. When she looks down, the face deforms, it looks like she was making a duckface :S
Because of that, the teeth keep coming out of her mouth when I rotate the head. I’ve searched a lot on Google, tried making the teeth a separate object (when I thought the teeth were the ones not moving properly) and played around with the weight painting near the mouth for a while before finally asking for help from the experienced ^^
The eyes, hair and teeth are separate objects, and the weight paint is completely red for the rest of the head. Any help is greatly appreciated.

Attachments




In weight paint select the other bones in pose mode and see what bone has influence on the face like the neck or arm bones. use the subtract to erase.
i think that is the problem.

Oh my god! How did you do know?! Haha. Thank you very much!
Indeed, I used “Automatic weights” to start and didn’t notice the chest bone was affecting all the way to the mouth.

and make sure the teeth are parented to the head bone