In all honesty, the whole face is flat due to lack of depth. Start with a sphere to help guide where the head is facing and as a guideline for where facial structures are supposed to wrap around the head. An understanding of basics form and anatomy will help your faces feel less flat. Even a loomis head can help be a guide for anime heads.
the sphere thing helps for sure. idk if the whole face is the issue though, the nose mostly reads flat because there is no shadow under it. maybe one small wedge of shade there and a soft plane on the cheek before going full loomis

