INTRODUCTION: In low-income and middle-income countries, it is estimated that one in every three preschool-age children are failing to meet cognitive or socioemotional developmental milestones. Thailand has implemented a universal national developmental screening programme (DSPM) for young children to enable detection of developmental disorders and early intervention that can improve child health outcomes. DSPM implementation is being hampered by low attendance at follow-up appointments when children fail the initial screening. METHODS: Action research, using qualitative methods was conducted with 19 caregivers, 5 health workers and 1 chief at two Health Promotion Hospitals to explore the factors affecting attendance at follow-up appointments. Transcripts and notes were analysed using descriptive content analysis. Findings were then discussed with 48 health workers, managers, researchers and policymakers. RESULTS: The high workload of health workers during busy vaccination clinics, and inadequate materials prevented clear communication with caregivers about the screening, how to stimulate child development and the screening result. Caregivers, particularly grandparents, had a lack of understanding about how to stimulate child development, and did not fully understand failed screening results. Caregivers felt blamed for not stimulating their child's development, and were either worried that their child was severely disabled, or they did not believe the screening result and therefore questioned its usefulness. This led to a lack of attendance at follow-up appointments. CONCLUSION: Task-sharing, mobile health (mhealth), community outreach and targeted interventions for grandparent caregivers might increase awareness about child development and screening, and allow health workers more time to communicate effectively. Sharing best practices, communication training and mentoring of DSPM workers coupled with mhealth job aids could also improve caregiver attendance at follow-up. Engagement of caregivers in understanding the barriers to attendance at follow-up and engagement of stakeholders in the design and implementation of interventions is important to ensure their effectiveness.