The first was concentrating on Sunnydale when it wasn't having Bringers kill the potentials. And the Bringers were what its army consisted of at the time, so they couldn't exactly see Jasmine to get infected (if she would have had any affect on them at all anyways), and people would be easy to kill if they were all shiny and happy. So Jasmine's plan wouldn't really effect FE's plan in any significant negative way. And Sunnydale seemed relatively cut off for most of the season, so Jasmine might not have gotten to them through the media, and people weren't exactly flocking to go to SD, so it's entirely plausible that Jasmine's influence wouldn't have reached there. And since Jasmine's plan had apparently been so intricately weaved into the events of both shows, it's entirely possible that the FE's plan was forseen, and that they had a way to prevent it, had she not been exposed as a gross maggoty thing and had her head punched in by Conner. It's hard to say, since I'm still not sure how the timelines work out.