Why does ADHD medication suppress appetite for some people but not others?

Individual response to stimulant medications varies for a lot of reasons. Differences in metabolism, dose, specific medication and formulation, and individual neurochemistry all play a role in how strongly the appetite-suppressing effect (a known side effect of the dopamine/norepinephrine pathway these medications act on) shows up. It's a recognized, common side effect but not a universal, equally-intense one for everyone. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/