I assume you have a subscription. If so, Delete a car that you won't be using, and try downloading that car again. You also need to check for sufficient internal storage space. I find one needs twice the storage space for the update that is downloaded.
For instance, if the Car manufacturer's file is 1GB large, then you need 2GB of space for it to do its thing.
My other question for you is if you have tried another Internet connection?