Since I got this 3DS XL, I've been having horrible problems with the local connection. Online play is semi-perfect, but local is horrible.
I can be right next to someone, and they're streetpassing people, but I'm not. They can streetpass ten, and I'll streetpass maybe two, whilst we're in the exact same area for the exact same amount of time. I have streetpass enabled as well as spotpass.
In my Pokemon X version, trading and batling with other people locally is a pain. I have issues when it comes to trading, either getting Error Code 018-0000 (An error has occurred. Please try again later.) or "Failed to connect with the other player" or "connection was lost"...and the 3DS systems are right next to each other!
I have the U.S. Pikachu 3DS XL and have made no edits to the hardware. I have tried turning off the wireless and turning it back on. Once I connect to the internet wifi, I'm fine except for a few rare instances of disconnection! But if it's merely local, it's spotty to near unusable.