A sincere compliment paid to your sources will boost their confidence and increase their commitment to help you . Remember to acknowledge their expertise and praise their insights. Be persistent and patient. You must be prepared to rethink and rephrase questions, asking for the information in different ways. Perhaps the source can't answer your question, but has bits and pieces of data which you can combine to arrive at your answer.
People like to help nice people.
Let your personality come through in an interview. Because you call the source in order to get specific information, you may be tempted to race through a checklist of questions. Engage in a conversation instead of a rigid survey. The source will enjoy it more, so you'll get more cooperation . And you'll learn important information that you never thought to ask about .