Enjoyment in learning mathematics is often perceived to be a positive, desirable emotion in the learning process. However, the findings of this study indicate that it can act as a barrier to persevering in mathematical reasoning by reinforcing a focus on habitual behaviours and inhibiting self-regulatory behaviours. The study identifies implications for practitioners; children's limited perseverance in mathematical reasoning (PiMR) may be masked by expressions of enjoyment and willingness to keep striving. Hence, to notice children's barriers to PiMR, teachers may need to look for children's repetitious use of reasoning processes or limited movement between reasoning process rather than relying on emotion indicators or expressions of being stuck. The study focused on children, age 10-11, selected for their limited PiMR and sought to better understand the conditions when difficulties encountered in mathematical reasoning became barriers to successful PiMR. Data, collected in two English schools by observation and interview, related to children's cognitive and affective responses and their active goals. A tripartite psychological classification was used to analyse children's cognitive, affective and conative difficulties in mathematical reasoning and to analyse when these difficulties became barriers that children were unable to overcome.