This paper reports on a study that explored the use of cognitively challenging tasks with low-attaining mathematics students and in particular, their teachers' attempts at scaffolding. A major finding was that responding appropriately to scaffolding opportunities was challenging for the teachers. In this paper two main factors are discussed which impacted on the teachers' responses to scaffolding opportunities: teacher knowledge of appropriate tools and representations for particular mathematical concepts, and teacher response to student prior knowledge and understanding.