Your device's front camera is used to detect your facial expressions and movements for your avatar.
To improve your avatar's movement, make sure that your entire face is within the camera's field of view.
If there is anything blocking your face (ex: hair covering your eyes, glasses you're wearing), then your camera may not be able to clearly read your facial expressions and movements.
Make sure that your camera is able to detect your eyes and mouth to improve your avatar's movement.