Answered by Tanya, Hiring Expert at Merck & Co., Inc., on Monday, July 20, 2015
This is a very tough question to answer without knowing all of the pieces of this mystery. There a couple of scenarios that may be at play here, the other candidates could have had additional atributes that made them a more attractive candidate to the hiring manager versus the HR rep, which would mean that you are not doing anything wrong.
Once again without knowing all of the pieces my best advice moving forward would be to perhaps video yourself in a practice interview and request feedback from anyone around you who will give you candid feedback. Also try to research what type of interview they will be conducting whether it is behavioral or skill based, practice your answers.