Most women seem to try racking the slide by pulling the slide back. If they are right handed, they are often trying to do this with their left (weaker) hand. Have her hold the slide in her left hand and push the gun forward with her right/dominant hand. (reverse if left handed). This works so much better!!!
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I think this is the reason I have seen that so many women cannot rack a gun. I have weak hands and I haven't found anything I can't rack using the technique you described.
Some men can't rack guns either because they are using the wrong technique.