Given that multiple countries have recently adopted social work practice models in children’s services, it is striking that only a few studies have systematically analysed both the level of fidelity and potential implementation barriers and facilitators. The aim of this study is to provide an in-depth analysis of how and why the Reclaiming Social Work (RSW) model works in different settings. The study context was the implementation in Finland of an adaptation of the model, the Systemic Practice Model (SPM). This mixed-methods study evaluates 1) fidelity to the SPM and 2) the possible influencing factors. The results reveal high variability in the extent of fidelity in 23 implementation sites, and even among individual teams within the same site. A lack of clarity concerning systemic social work practice, insufficient training, and inadequate resources and leadership hindered the implementation, whereas coaching and positive experiences of the SPM were facilitating factors. In particular, the involvement of a clinician qualified in systemic family therapy was crucial in embedding the new approach. The relationship between the level of fidelity and the influencing factors worked both ways (e.g., low coverage was associated with a decrease in participant responsiveness, and vice versa). Given the complexity of children’s social care as an implementation environment, careful preparation and ongoing support are crucial in the implementation of practice models.