Social capital is increasingly conceptualised in academic and policy literature as a panacea for a range of health and development issues, particularly in the context of HIV. In this paper, we conceptualise social capital as an umbrella concept capturing processes including networks, norms, trust and relationships that open up opportunities for participation and collective action that allow communities to address issues of common concern. We specifically outline social capital as comprising three distinct forms: bonding, bridging and linking social capital. Rather than presenting original data, we draw on three well‐documented and previously published case studies of health volunteers in South Africa. We explore how social contexts shape the possibility for the emergence and sustainability of social capital. We identify three cross‐cutting contextual factors that are critical barriers to the emergence of social capital: poverty, stigma and the weakness of external organisations' abilities to support small groups. Our three case studies suggest that the assumption that social capital can be generated from the ground upwards is not reasonable. Rather, there needs to be a greater focus on how those charged with supporting small groups—non‐governmental organisations, bureaucracies and development agencies—can work to enable social capital to emerge. Copyright © 2014 John Wiley & Sons, Ltd.