This paper describes a study conducted with a random sample of 80 student primary teachers drawn from all four years of the Bachelor of Education (BEd) programme at a teacher education institution in Scotland, with a view to determining why there were such differing levels of engagement with an online maths assessment. The assessment was created in an attempt to address deficiencies in subject knowledge in order to reduce the amount of time spent on mathematics remediation, and to raise awareness of the levels of mathematics competence required in the primary classroom. Study of the reasons behind the differing patterns of engagement with the assessment revealed that two thirds of the group were able to reach a competence threshold and often to improve upon it by some way; a worrying third of the students, however, made little attempt to use the tool to improve their subject knowledge. A further finding indicated that students who engaged with the online assessment reported improving levels of confidence in mathematics. (Contains 6 tables.)