Board Thread:Questions and Answers/@comment-27801808-20160215082214/@comment-27701762-20160217022851

Cheeseskates wrote: hmmm. did you forget to  hang out with everyone, like, go back to papyrus' house after his fight? same with undyne after their fight?

if you do those, you should get the ending properly after beating asgore again,and backtracking to MTT resort. Right. You need to "befriend" three characters: Papyrus, Undyne, and Alphys (in that order). After fighting Papyrus, you can befriend him. Just head to his house and you do a little "date." Once that's done, you can befriend Undyne. Go to her house. From there (since you've already completed the Neutral Route), you'll get a prompt that should guide you toward the True Pacifist end (I'm trying to avoid explaining any more at this point to not spoil what happens).

If you've already hung out with both Papyrus and Undyne prior to the Asgore fight, then as Cheeseskates said just head back to the MTT Resort, and on your way there you'll get a call that will explain what you need to do.

Doing the Asgore fight over again will do nothing but just give you the same Neutral ending, and will skip the fight with Flowey, since he knows that if he does the fight again the souls will rebel.