I totally agree with CUkayakGirl regarding athletic ability and focus. I've only been boating 5 and some years, but from what I've seen, flexibility, which usually relates to things like athleticism, age and past injuries, gender and maybe more, is key.
Another factor is a person's level of comfort in the water, which in turn affects stress levels when upside down, which directly results in tense muscles and lack of flexibility.
A trait of athletes is the ability to focus on achieving a goal. Athletes are naturally obsessed to the point of mastery, be it on a personal or professional level.
So far, the best new rollers I've witnessed were athletic, water loving, young men. A new female co-worker, who is all of the above, wants to learn to yak. I gave her EJ's R&B and Kent Ford's the Kayak Roll last week. She's meeting me at the pool this weekend, so I'll see what happens. Hopefully the teacher won't be another factor to consider.