I got both the black and white 2 games and started trading them back and forth via infrared connection, I also put both the cartirdges in both the ds's (one at a time) and since that happened I cannot sync via connections. I changed the dates on my old ds because it was wrong but as for my 3ds it's correct.
Please help before we do it again; lol.
make sure they are both in trade .
try turning them both on at the same time.