It's hard to explain, but you have to learn to translate your design from paper to a real form. It is pretty easy to make a shoe look dope on paper or on your screen. You can cheat proportions and make the shoe look like it is hugging your foot and it is lightweight and it is fast and it is any other adjective you can think of to describe an emotional design. But that's just it; you have created an emotion that looks good in two dimensions. Now you have to capture that same emotion in three dimensions. Mike Tyson said it best: "Everybody has a plan until they get punched in the mouth." Well, everyone has a beautiful design until they see it in 3D. The quicker you learn how to sketch in 3D, the quicker your 3D form will be beautiful.
Learn to Think in 3D