Both in New Zealand and internationally, there has been a focus on the use of differentiation in mathematics instruction to raise achievement levels and provide equitable outcomes. New Zealand has a long history of the use of ability grouping to provide differentiation. Recently, this practice has been challenged in a large scale professional learning and development (PLD) initiative entitled Developing Mathematical Inquiry Communities (DMIC) which focuses on shifting to strength-based, capability focused heterogeneous grouping practices. This article draws on a case study of five teachers involved in the DMIC PLD initiative to examine the enabling factors and barriers to shifting teacher beliefs in relation to mathematical ability grouping. The findings indicate the persistence of teacher beliefs related to fixed ability levels in mathematics. Assessment practices focusing on narrow domains in regard to success in mathematics appeared to act as a barrier to changing teacher beliefs. In contrast, a focus on strengths that students brought to a mathematical task and collaborative group-work appeared to be a key enabling factor to change in teacher beliefs and practices. A key implication of the article is that changes to assessment practices are required alongside changes to pedagogical practices to support teachers to move from gap gazing to recognising multiple strengths in the mathematics classroom.