I started very recently in the DL. Specifically in object detection I am starting my first projects so I have almost no experience.
I think the model takes into account the tires, the mirrors, etc. to identify the cars, and it is not very far from what we are looking for. If you could increase the set of images (it is the first thing I would try), then regulation techniques. I don’t think you can add new images since you are using a DB from Kaggle. Are you on a Kaggle competition or just downloading and using the images on your PC?