To evaluate the failed implementation of the Carer Support Needs Assessment Tool Intervention for family caregivers in end-of-life care, within a trial context using Normalization Process Theory (NPT). An evaluation study was conducted to learn lessons from our trial, which was not successful due to the low number of participants. The evaluation study utilized various data sources, including published data from interviews and questionnaires, and unpublished data derived from emails and conversation notes. Data were retrospectively collected. Thematic analysis was conducted guided by the NPT framework. This framework emphasizes that successful implementation of an intervention relies on its 'normalization', consisting of four constructs: coherence, cognitive participation, collective action and reflexive monitoring. Coherence (sense making): Nurses felt the intervention could contribute to their competence in assessing family caregivers' needs, but some were unsure how it differed from usual practice. Cognitive participation (relational work): Nurse champions played a crucial role in building a community of practice. However, sustaining this community was challenging due to staff turnover and shortages. Collective action (work done to enable the intervention): Nurses felt the Carer Support Needs Assessment Tool training enabled them to improve their support of family caregivers. However, contextual factors complicated implementation, such as being used to a patient rather than a family-focused approach and a high workload. Reflexive monitoring (appraisal of the intervention): Positive experiences of the nurses with the intervention motivated them to implement it. However, the research context made nurses hesitant to recruit family caregivers because of the potential burden of participation. Although the intervention demonstrated potential to assist nurses in providing tailored support to family caregivers, its integration into daily practice was not optimal. Contextual factors, such as a patient-focused approach to care and the research context, hampered normalization of the intervention. Assessing and considering contextual factors that may influence implementation of a complex care intervention is needed. The NPT provided a valuable framework for evaluating the implementation process in our study. What problem did the study address? This evaluation study analysed the factors that promoted or hindered the implementation of a nursing intervention to support family caregivers in end-of-life care. What were the main findings? Both the intervention and the intervention training have potential and value for nurses in providing tailored support to family caregivers. However, the implementation faced challenges due to organizational factors and the research context, including recruitment. Where and on whom will the research have an impact? This insight is valuable for all stakeholders involved in implementing complex nursing interventions, including researchers, nurses and funders. This study has adhered to the relevant EQUATOR guidelines: Standards for Reporting Qualitative Research (SRQR). There was no patient or public involved. The trial was prospectively registered on the Dutch Trial Register (NL7702).