I just wanted to add aside from the correct answer from Joe, I too have been having issues with the street pass function not working 100% of the time. When I first received my 3ds I would go over to my brothers house and it would instantly light up, but tonight when I went over it took a while and a little fiddling with to react. My honest opinion would be that it is still working out the little bugs and most likely be honed to perfect with a ninja update. Still a lot of features for this system to pop up.
My advice would be to just set the two down next to each other and let them sort it out.
Also, 100% correct on the "cooldown" for the data swapping, 8-10 hours. Although I haven't tried getting one swap, going out to get a different one, and then trying to swap again. That might be worth an experiment. Maybe you can't get the same data unless you get different data.. following me? Alright I'll go crazy professor laugh over here by myself.
Hope this helps.