You gotta make them ENJOY the moment, not only put them on that action
Try hypno+making them feel good on sex( maybe throw a fetish pilll or something idk)
Example: the masoquism fetish only goes up if either the npc cums or at least has more than 70% of satisfaction when ended, with that in mind you just need (as stated before) to raise their lust via hypno, so that they can cum more, resulting of course, to enjoy it more
Maybe this was pure yap and got some facts wrong idk, but this is how i mostly raise fetishes on npc to an acceptable status