Vampires / Feeding Questions

So my vamp sim has made her rounds so to speak on her quest to find out which men in her sim world are yummy sims to her, so she is at least acquaintances with pretty much every guy there is. In most cases she was able to get the guy to agree to allow her to drink, but a few are keen to deny her even when it gets to a pleasant conversation (Johnny Zest and Joaquin Le Chien are especially prone to saying no, I guess they have vampire phobias or something). In Joaquin's case, it probably doesn't help that my vamp turned his roommate Sergio so he is now living with a vampire. I am not sure if the game randomly assigns certain sims to be less likely to agree, or if it is something that randomly happens every so often, but both of those guys have rejected her request twice now (with pleasant conversations). But I guess those aren't bad odds compared to all the victims who agreed so I'll take it.