Here's the list of all 24 items you get from the New Driver Challenges, can you check your inventory and confirm whether you got them all? (Sorting your Inventory by Most Recent and filtering for Common Quality should make it easy.)

One other person mentioned the same problem, but what I suspect may be happening is that once you complete them all, they automatically unlock without you having to claim them. But that's just a guess, and I can't check that myself. However, the Weekly Challenges just updated 2 hours ago, so maybe it's a problem related to that.

If you are missing any of those 24 items, contact Rocket League Support: http://support.rocketleague.com/