i play mania at school and have to use school membrane keyboards and i hit hard and i have loads of stamina but when i get home and hit notes gently on a mechanical keyboard suddenly my stamina disappears and it pisses me off
Observe yourself on how you play on a membrane keyboard versus a mechanical keyboard. If you notice some differences then that'll give u a better idea on why your stamina is worse on a mech keyboard. Other than that, theres not much information to go of apart from general advice like not slamming the keys.