Over the last 25 years of doing systems, the biggest things I've run into is being the Bad Guy, lack of cooperation and being Led Off-Course.
Don't take it personally or get mad at anyone...
Many companies don't inform business units and users of upgrades to their systems....so when suddenly, I try to contact a manager in a department or a user to get some information on their workflows, etc....many will think I'm only there to design a system to eliminate their job....suddenly they will not be available to you and some will even try to give you bad information, thinking it will give them some extra time before being laid off.
Depending on the size of the project, we will usually try to break the ice with a Project Kick-Off meeting...with the departments / users involved and some food and beverages....explaining what is going on and introducing the team members they may contacted by....and then answer questions to ease their minds. Someone from upper management is usually there to give support to the team and answer those "Will we be laid off" questions. Once the ice is broken, it is usually much easier to work with people.
Also
Don't get technical with people....don't try to impress users with technical Jargon....sometimes they will feel embarrassed and don't want to talk to you anymore for fear of being embarrassed again.
Also
Sometimes you will have to interview a manager that is so caught up in themselves....telling you how important their job is and how they are putting aside valuable time to meet with you and your very presence in their office better be worthwhile....Blah, Blah, Blah.
Just sit there and listen to the lecture and when they are done, thank them for making time to meet with you....and start asking your questions.
Gee, there are so many things....every day is a new day.