Breast milk substitute (BMS) marketing harms breastfeeding and public health. To control BMS marketing, the Member States of the World Health Organization is called upon to adopt all provisions of the International Code of Marketing of Breastmilk Substitutes (the Code) into national law. In 2017, Thailand adopted many provisions of the Code through the Control of Marketing Promotion of Infant and Young Child Food Act B.E. 2560 (the Act), including the establishment of a compliance monitoring system and enforcement mechanisms. Nevertheless, recent research showed widespread violations. This study aims to assess gaps in the monitoring system and the Act's enforcement in its first three years of operation. This qualitative research study employed in-depth interviews between April and June 2020 with 34 key informants (KIs) from the Thai government, academia and civil society organisations. KIs identified gaps in six areas that could be mitigated to increase compliance with the Act. These gaps are unclear provisions on coverage of the Act; communications with retailers and the public; lack of strong direction and processes; inadequate budget allocations; skilled and confident human resources; and external factors which facilitate BMS marketing. Recommendations may be relevant for other countries and include revising and clarifying the Act, developing targeted communication strategies; providing clear monitoring direction including through setting key performance indicators related to the Act; and providing sufficient budget and training for authorised and support officers. Strengthening the health system and workplace support for breastfeeding and social marketing would also help address wider structural factors.