Hi Bonnie! Thanks for your question, and happy to help explain this
Your prephrase was actually correct! The flaw in the argument is that the skeptic doesn't consider that Debbie may have switched her method during each of these trials. So maybe Debbie did use sleight of hand, but she did so during the second trial and not the first, which is why the videotape didn't find sleight of hand.
However, your prephrase matches answer choice (A), the correct answer, rather than answer choice (B), the incorrect answer. To see why, let's go through what answer choice (B) means. If answer choice (B) is corect, that means that the flaw in the argument is that the skeptic did not consider the fact that sleight of hand could have been detected by another means. However, this is not the flaw in the argument. Just because sleight of hand could be detected another way does not mean that the skeptic could not have detected it using videotaping, as he tried to. Furthermore, he may have used a different means and still not detected sleight of hand because she changed her method during that trial. So the issue is not that he chose a poor method of detecting sleight of hand, but that Debbie may not have used sleight of hand when he tried to detect that method.
The issue with the argument is that the skeptic concludes that none of these methods, not just sleight of hand, were used, without considering whether Debbie switched her method during each trial. This is exactly what answer choice (A) says, and this answer choice also matches your prephrase. If she did not always use the same method, then this is why the skeptic's trials failed to detect her method, because he was specifically testing for one method during each trial. Answer choice (A) accurately portrays this possible switch of methods by saying that the skeptic failed to consider that "Debbie did not always use the same method to achieve her effect."
To sum up, your prephrase was correct! The issue with the skeptic's argument is he fails to consider that Debbie may have switched her method during each trial, which is captured by answer choice (A). Answer choice (B) describes a different flaw and is thus incorrect.
I hope this helps, and let me know if you have any other questions!