I've been experiencing this, as well. I tried sending gifts to people, in attempt to get different items in return. I'm not really sure why, but I sent a letter to Bob yesterday, without a present, and he sent me a gift back with a nice letter. I sent him a letter back today with a gift attached, though.
It feels like it’s taking forever
but one day, things can get better
then maybe my time will come
and I’ll be the lucky one
I'm not entirely sure if this is the same in New Leaf, but it seems to be working for me. In the previous AC games, sending a letter that was written as so: "Hi! How are you? Good bye!" with a present attached, always guaranteed a return gift. So far, I have always received a present from my neighbors.