Are the cars not available to buy?
Are you trying through I game marketplace or the main Xbox one? I bought season pass and everything seems to be straightforward so far.
Oh eck! Did you pay through xbox live marketplace or in-game via Forza marketplace?
Download cars (again) from in-game car packs - the Nov DLC pack should show as FREE if you have paid.
Failing that ... the only thing I suggest is go to download history and download again. Fingers crossed.