Being a product-focused Mechanical and Software Testing Engineer, I walked the interviewers through a valid troubleshooting method I would take, troubleshooting everything from the electronics to the mechanical parts, and from the front end to the back end. They didn't understand my answer and asked about the user perspective. Even after going over every possible action a user could take they just moved on. I understand that this isn't an engineering position, however, the interviewers were unable to communicate the information they were trying to get, as I answered the question as they worded it as best as it could be answered by someone not in the vending machine troubleshooting field.