I have just recently purchased the Lamborghini Reventon (single purchase, not with the car pack) and it said that it has downloaded but there is no free sign and the dlc sign has also gone. I’ve checked my garage and the messages section and there is nothing, I need help because it cost me money 
Shut down your console, unplug it from the outlet for 10-20 seconds, and then boot back up. DLC purchases don’t automatically put the car in your garage - you have to go to the Buy Cars menu and buy the model. The first time you buy the model it should cost no credits.
After you boot back up and attempt to buy the car model from Buy Cars, is the game prompting you to download the pack again? Or is it allowing you to buy it but asking you for credits or tokens (and not providing the one free car model)?
Same thing just happened to me. I purchased the Nurburgring Booster Pack and none of the cars show up as “free” when I go to buy them. I shut down the system, unplugged it, restarted and still nothing. It’s asking me to use credits or tokens to buy just like with any other car.
I have just purchased the new dlc hot wheels and now when I go to my garage my cars from all the other car packs dlc i have purchased are in there but to use them it asks me to pay cash again to use them ?Is this to do with the update ?
MODERATOR NEEDED
Perform a complete shutdown and reboot of your console. Hold the Xbox One power button (not the controller) until the console turns off. Then, reboot.
PS - You don’t need to spam multiple threads with the same post.
That did not work for me.
But I am happy to report that after emailing Forzafb@microsoft.com my problem was fixed. 
Thank you T10!