Also something that might be a problem when drawing with a face - when you're drawing it just as practice and drawing loads, you're not as worried about how they'll look, and you'll be drawing looser and more naturally - when you actually go to draw a proper picture, you want it to look great, so you're much more focused and aware of every line you're drawing, so you may be stiffening up, making it harder to make natural looking lines. if you try and relax, and just treat your picture as another practice, it may help a bit but definitely check out what rubisko mentioned, having a properly structured face will help too