Competency-Based Curriculum (CBC) reforms have been adopted across many education systems, particularly in sub-Saharan Africa, as a strategy for shifting instruction away from rote content coverage toward the development of learner competencies, skills, and applied knowledge. Despite the policy appeal of CBC, a persistent gap has emerged between what national curriculum frameworks prescribe and what actually occurs in classrooms. This systematic literature review synthesizes evidence from peer-reviewed studies, government reports, and grey literature published between 2013 and 2025 to examine the institutional and systemic barriers that constrain CBC implementation. Following PRISMA guidelines, 54 studies met the inclusion criteria after a multi-stage screening process from an initial pool of 412 records identified across four databases. Thematic synthesis of the included studies produced six dominant barrier categories: policy design and communication gaps; institutional capacity and resource constraints; teacher preparedness and continuous professional development deficits; assessment and examination system misalignment; weak leadership, governance, and monitoring systems; and socio-cultural and contextual resistance. The review finds that policy-to-practice gaps are rarely attributable to a single point of failure but instead emerge from the cumulative interaction of under-resourced institutions, examination systems that still reward content recall, inadequately supported teachers, and top-down policy processes that exclude implementers from design. The review concludes that sustainable CBC implementation requires coherent alignment between policy intent, institutional capacity, and classroom-level support structures, and proposes a multi-level framework for closing the policy-to- practice divide. Implications for policymakers, curriculum developers, teacher training institutions, and future research are discussed.