One of the main things that's stifling the rendering of the forms is the lack of contrast. There should be a drastic enough distinction between the roof of the car, the hood of it, and the bottom platform in the back. Hue shifting might also come into play to help that effect. With effective highlighting, it will push the forms to seem more 3 dimensional. (For example on the hood of the car because it protrudes more, the roof of the car and the back wall.) (which it looks like you have a slight highlight there but can be pushed much further with the right colors.
I feel like the front bumper of the car should also be curved a bit more so the very front protrudes more. About the back wheels, yours are more crooked and diagonal whereas your ref has them straight on, so that could be part of the problem you're having.
I think this is a neat idea, hope my comments help.