Ok, I did everything people here in the comments said to do to find Sans and Grillbys, but nothing worked. I tried to get lost, I did the forest part the right way, I tried to use a different soul and changed my choices... but it never works!!!!
Sans only shows himself to you if Papyrus likes you, and you'll only get the offer to Snowdin if Sans likes you. To meet with Papyrus you have to linger in waterfalls. :)
