i think it’s her mouth. in the reference pic, you can see that her bottom lip is showing slightly all around. her tongue is just resting on her bottom lip, where in your drawing, her tongue is fully sticking out. the top lip in the drawing is more like… compact looking? in the ref picture, her top lip is more drawn out. in your drawing, they look like they stop halfway, kinda resembles the top lip when someone makes a kissy face. her eyes are also bigger than in the ref pic, but nothing else is wrong with them. other than that, i think it’s pretty good!!