What Do I Do if An Interviewee Is Unresponsive to Interview Requests?

What do you do if an interviewee is unresponsive, and I don't mean like they're no longer living, what if they won't talk to you about setting up your interview, they're ghosting you? Maybe you complete the interview with you and then they don't follow up with the documents they promise to send over. At the end of the day, and this is really hard to hear by people who want to get answers: you document all the times that you tried to get the information. It could be that the best you can tell your client is "Tommy told me that he had three emails that would establish the underlying evidence related to his concerns. And I asked him eight times for it, I called him four times, emailed him four times and he never sent over the emails." As the investigator, I just document the attempts to collect information. It sure does go to his credibility because he told me there was all this stuff that was going to prove what he had to say and he didn't provide any of it. But again, as the investigator... That looks really bad on the employee because their employer expected them to participate and needed them to participate in the investigation and sometimes employees don't participate, that kind of comes back on them. But we need to also document that we gave them the opportunity to participate.