I was recently hired for one of my first new projects and have been interviewing stakeholders. I got through the interviews with other people who work for the client, with no problems. However, the people I could interview are all other people who work on the site. They use the site from a contribution/moderation basis; they are not their target audience.
When I tried to interview external users, some of the client management told me, "You can't talk to them. Only we can." The people saying this are not UX people, so they do not know the right questions (or follow-up questions) to ask. I had already emailed many of these external users to ask for the interview and was only told this after the fact. The users I am trying to interview have a lot of influence in their organizations and can reach or speak for many other potential users. All I've been able to do with other external users is launch a quick survey, which should need a lot more respondents to really become useful.
What should I do to handle this situation well?
Edits suggested by @JohnGB:
My client is a volunteer organization that has a lot of youth participation. They've had some problems with some of their participants being rude/uncivil about and toward these other organizations. In effect, they're telling me I can't talk to these people directly as a way to protect them. I just think it's keeping me from doing my job as well as I can. The other organizations have huge user bases that I can access by interacting with these people.
My client is looking to expand their user base and social media reach. They maintain about 8 Facebook pages with ~4000-5000 likes among them. Most of the existing users match one of the personas I anticipate for the design. The other organizations have hundreds of thousands, or over a million, Facebook likes each. They represent a better cross-section of the general population in their areas. There's really good potential if I can reach that part of their target audience effectively.