I had villagers ping me to move when a Peanut's house plot appeared before I think. o:
I think either:
1) You probably can't find the plot but it's somewhere in town.
2) You got a VOIDED villager/villager from streepass when you were asking her to move in, meaning she couldn't moving in since you got a voided/streetpass villager attached to your town already.
3) Maybe you didn't actually ask her to move in? Do you remember the conversation? Sometimes you have to talk to a villager 3-4 times for them to move in.
4)Her house plot is just lagging for some reason, just continue tt-ing until Isabelle announces that a new neighbour is moving in (aka Whitney)
Hope this helped somehow. o: