OBJECTIVE: Immunisations are highly impactful, cost-effective public health interventions. However, substantial gaps in complete vaccination coverage persist. We aimed to describe caregivers' immunisation experiences and identify determinants of vaccine dropout. DESIGN: We used a community-based participatory research approach employing Photovoice, SMS (short messaging service) exchanges and in-depth interviews. A team-based approach was used for thematic analysis. The Increasing Vaccination Model guided the analysis and identification of vaccination facilitators and barriers. SETTING: This study was conducted in Zambézia province, Mozambique, in Namarroi and Gilé districts, where roughly 19% of children under 2 start but do not complete the recommended vaccination schedule. PARTICIPANTS: Participants were identified through health facility vaccination records and included caregivers of children aged 25-34 months who were fully vaccinated (n=10) and partially vaccinated (n=22). We also collected data from 12 health workers responsible for delivering immunisations at the selected health facilities. RESULTS: Four main patterns of barriers leading to dropout emerged: (1) social norms and limited family support place the immunisation burden on mothers; (2) perceived poor quality of health services reduces caregivers' trust in vaccination services; (3) concern about side effects causes vaccine hesitancy; and (4) caregivers hesitate to seek and advocate for vaccination due to power imbalances with health workers. COVID-19 created additional barriers related to social distancing, mask requirements, supply chain challenges and disrupted outreach services. For most caregivers, dropout becomes increasingly likely with compounding barriers. Caregivers of fully-vaccinated children noted facilitators, including accompaniment to health facilities or assistance caring for other children, which enabled them to complete vaccination. CONCLUSIONS: Overcoming immunisation barriers requires strengthening health systems, including improving logistics to avert vaccine stockouts and building health worker capacity, including empathic communication with caregivers. Consistent and reliable immunisation outreach services could address access challenges and improve immunisation uptake, particularly in distant communities.