The earliest difficulty you may encounter with the Elman induction is that you client opens their eyes following your suggestion that they stay shut. There are a number of reasons why this may happen.
Firstly, they may simply not have understood your suggestion properly. It is reasonable that they believed you expected them to try as hard as they could to open their eyes, so they did. They went right ahead and opened their eyes! This situation is easily rectified by saying something like, “Well done. You’ve demonstrated that you can open your eyes. As I said, you remain in control at all times. Now, I want you to demonstrate that you can’t open your eyes. So, go ahead and close your eyes again and relax them so well that they are just too relaxed to open. And then demonstrate to yourself that you’ve relaxed them that well by keeping hold of that relaxation whilst you try in vain to open them.”
A second reason that someone may open their eyes is fear. Despite a positive pre-talk, they may still equate not being able to open their eyes with being controlled by you. In that case, you can proceed with a more permissive version of the eye-lock, or use something other than their eyes.
Some people really benefit from being able to see their own catalepsy. So, you might choose to have them watch, whilst their finger stays glued to their leg. However, if you do move on from an eye-lock to something else, it is important not to give the impression that they have failed. Say something like, “Good. You’ve demonstrated that you are able to stay in control, just as I said you would. And now I would like to use that powerful mind of yours to show you something really interesting…”
I would argue that failure to understand and fear are the two main reasons that someone may open their eyes. Another reason this sometimes happens is that someone feels like you are challenging them and they do not like to lose. In that case, simply reframe things as above.