I don't think anyone really knows. Have you missed any DLC yet? Try leaving your 3DS in sleep mode over night in the charger dock (that's how I got my SpotPass content) and see if you get either of the 2 missed DLC items. I've had the game since release and don't normally turn off my 3DS so I haven't had any issue getting the content.
I think it's best to see if someone who just got the game gets both the item and the level.
Current goal: Purchase all the 3DS games I want on release and have the best 3DS collection ever!
I still never got my second spot pass(the stage). I leave my 3ds on sleep mode every minute I'm not playing. And I make sure my wi-fi is on. I'm currently still waiting for my spotpass, but honestly I've already lost hope.