You must constantly move back and forth between the Room with the ENTRANCE of Nebby's shop (DO NOT ENTER HER ACTUAL SHOP) and the Hallway with Chara's room. It's a very low chance of happening, but you will eventually be teleported to the area below.
(Yep I copy from Phaltar :) so you should find his/her comment somewhere, and please don't "bonk" me)