Teachers' diagnostic competence is fundamental for supporting students' individual learning processes and must be fostered in teacher education. Following a design-based research approach, a learning environment is developed focusing on preservice physics teachers' diagnostic competence in diagnosing students' conceptions. A core element of the learning environment are video vignettes of high school students solving tasks on mechanics. The learning arrangement was implemented in a bachelor's level physics didactics course. We aimed to identify supportive and obstructive elements for the preservice teachers' learning processes when training with the video vignettes to inform the redesign of the learning environment. The working phases centred around the training video vignettes were videotaped and data of four groups of preservice teachers was analysed with thick description. We reconstructed preservice teachers' intentions of action to gather a deeper understanding of supportive and obstructive elements. Among other findings, results show that preservice teachers' gaps in content knowledge highly influence the training situation.