Glad to hear you could do streetpass. To be honest I ran into the same issue about a couple of months ago after getting a New3DS; after some time both the 3DS I had from before and this New3DS I bought recently stopped encountering each other after the typical ~8-hour interval. It's pretty much the case that Pluupy mentioned; 3DSs don't like being too close to each other for long periods of time to essentially "abuse" the streetpass function, so they just stop encountering each other until some long time passes or you encounter other people.
Even so, I consider that recent updates deteriorated the Streetpass function as time passed (of course, the moment Nintendo stops paying attention to a feature in their consoles, they basically condemn it to death unless something unofficial keeps it alive), mostly because I noticed that even on "big" 3DS meetings I attended during the last years the amount of encounters the console detected dropped significantly with every instance in question... not to mention, the dropping of Streetpass hotspots that happened last year must've reduced the encounters even more.
It's truly a shame, but then again it's Nintendo the one behind it so sadly it was pretty much bound to happen...