Hi. I'm pretty new around here as well, but have been on IP for 3 months now. I have had weeks of big weight loss (not 15 pounds though) and others of only about 1 pound. I am down 49 pounds, so averaging 4 per week, but that is really all it is - an average.
Given the limited choices you have with your soy sensitivity, I don't know how you could follow the protocol any more closely. You may want to take some body measurements each week along with stepping on the scale. Sometimes when the scale doesn't move much, the tape measure tells a better story.
Best of luck to you and your wife on the plan.