Djtooth Depends on...well - pretty much every mechanic in the game. High virtue characters, for example, would require lover status and a date before they become "willing". Others might hate you after a bunch of failed interactions, which snowballs into more failed interactions. You might as well hit the wiki to find out the "why?"s and "how?"s with how many factors can be the cause.
One of many tips from that same wiki, for example: "When there is a 0% response, there is a fundamental problem, so change your tactics.". Meaning, either the character hates you, has preferences you do not match, or something else, or you used the same interaction on them 3 times during the same period. And so on.
P.s. Lowest virtue characters require no spam - they have a hefty chance to agree to have sex on the very first interaction, without any prior relationship building. So, are you expected to spam? "It depends" on the character you go after.