You could also try taunting her with noises when you tickle her. My ex used to find that extra ticklish.
Like I said though when it comes to her feet you have to get through her mental this doesn't tickle barrier. It obviously does 😉
Don't let her mind get used to any spots either, if her soles are ticklish tickle her there briefly, then go for her toes, then back to her soles again or maybe her heels. You get the point, don't give her a chance to get use to the tickles. 😉