Identify the barrier. The first steps may require some digging. If you are not getting the information you need, there could be multiple reasons, such as legal concerns, respondent availability or respondent reluctance. It's also possible that your data collection technique could be the issue. Consider asking the respondents for more information or asking a colleague to review your questions or methods. Ask a lawyer or ethics expert if the information you seek is privileged in some way.
Review information already available from other sources. Collect information already published on your issue, or about the group of people (or other subject) of your inquiry. For example, population or demographic data could be available for the surrounding counties, if not for your county. You could ask other researchers about their experience collecting data in this field.
Try alternative methods or sources. There are many ways to collect data, and each requires different techniques and resources. Look for another way to ask the same question; this could lead to more fruitful data. If you cannot access a group of people, or if you lack a technical solution for the object you're studying, try another group or object that is similar.
Consider hiring an external researcher or assistant. There are a couple advantages to this. First, they may bring new skills or techniques to the data collection process. Second, they may have a different rapport with the respondents. Interpersonal dynamics between the interviewer and respondent can be improved by introducing someone new.
Try a different, but related question. A respondent will not admit to something, such as illegal activity, even if you guarantee confidentiality. Ask about other, related pieces of information that will help complete the overall puzzle.
Recognize your limits. Time and money are precious resources when collecting data. There are always unforeseen challenges requiring some creativity. Ultimately, the information you seek may not be available with the resources at your disposal.