Connection error with download game Nintendo 3Ds / Ds?

I and my sister wanted to play a game from the DS (Marioparty) together via "Download-spiel" (she on the DS and I on 3Ds). My Nintendo also shows her game and you can see my name on her, but unfortunately tretted I got a connection error every time. Would be nice if someone responds quickly.

You can't be too far apart.

Our Nintendo's were actually very close to each other.

See if there are connection problems with other games as well.