Despite progress with national adaptation policies, the adaptation deficit is growing. Barriers to adaptation are a key reason for the adaptation deficit. Past research on barriers only offers a conceptual understanding of the barriers, with limited insight into real adaptation processes. Common causality, interrelationships and dynamics of the barriers remain under-researched. Examining the cases of South Korea and the UK, this research aims to improve our understanding of the common barriers to national adaptation policy. Drawing from semi-structured interviews and documentary material, we identify 54 common factors in the two countries (17 barriers, 17 origins, 20 influences), and eight key barriers: conflicts between government departments, frequent rotating of civil servants, lack of political will, unclear range of participants in national adaptation policy, low priority of adaptation, uncertainty about the effectiveness of adaptation policy, differences between adaptation and election timelines, and lack of understanding of adaptation. We explain the origins and links between factors and the common causal mechanisms of barriers to national adaptation policy, as well as their influences by mapping them. Based on the mapping, we argue that there are barriers that are easier to address than others, and that there is a need to focus on them to reduce the adaptation deficit effectively. We conclude by discussing the policy implications of our findings.