I've had this happen a few times with smaller follicles. Now, I've only been somewhat successful with palpating (finally got the hang of it last season), so I'm definitely no expert. What I experienced was that with smaller follicles, about marble size, I would feel the follicles the first time, try for a second time to make sure I counted right, and sure enough, didn't feel anything. A few days later I tried again and sure enough, I felt the follicles again. It might just be her tensing up. Once the follicles get bigger, its easier to feel them each time.
just as jay said I would wait a few days and palpate again. let us know how it goes chances are she did just tense up and at that size they'd be easy to miss.