My friend and I tried it through Mii Maker, we've tried it through Smash Bros, and yes, we have copies of both versions, and we've tried it using two different 3DS systems, neither connected, when it did so just fine before, we were able to transfer Miis just fine.
For Mii Maker, after searching for players, the Wii U would appear on the list, but after I tap it, it would almost immediately say a connection error occured.
For Smash Bros, it does pretty much the same, it'll say (Wii U name) is available, after a short attempt at connecting, it'll say "Unable to connect."
I tried everything I can think of, putting the 3DS right next to the Wii U, turning both consoles off for awhile, I even tried lifting the thing up . (The console used to be placed higher up... I dunno, I got desperate.)
Both the 3DS we used are fine, the Wii U hasn't been moved much.
Is there just something I'm missing...?