Those who produce it may not be interested in the same questions as sociologists, and so secondary sources may not provide exactly the information that sociologists need
Sociologists are influenced by the values of the society they study and thus are influenced by its values. As these values change, so does the focus of research. The rise of feminism in the 1960s led to a focus on gender inequality and today's environmentalist concerns have generated interest in 'greencrimes' such as toxic waste dumping.
Inaccessibility of certain situations to the researcher may restrict what topic they are able to study. For example, although sociologists may wish to study the ways in which global corporations make their decisions, this may not be possible because these are made in secret.
Collecting a sample by contacting a number of key individuals, who are asked to suggest others who might be interviewed, and so on, adding to the sample 'snowball' fashion