I think that failure in escalation is often the result of not softening up the target enough beforehand. The jump is too abrupt for her.
I will usually soften the target by doing things like walking through every doorway and stairway right next to her, crowding her and forcing her to squeeze through together with me. Of course I blame her and ask, teasingly, "Why don't you let me pass through? A classy lady would let me pass through." The more astute of you will realize that I'm planting a phrase that I want her to reference later: "Let me pass through." The combination of the boldness, the audacity and the physical closeness will tend to thrill her, and will leave her much more open to escalation later. I will also gently push them into trees, posts, etc. (as if there was no room for us to go past the obstacle together), telling her she should be more careful, but not to worry as I'll protect her. She begins to associate your bodies touching with fun and pleasure.
These are also good opportunities to jokingly imply she is a bit clumsy and needs my help ("I have so much work to do with you and so little time").
I find that subtly setting the stage as early as possible with "innocent" and fun touching makes them very open to physical escalation later.
I always get them into a state where they're dying to escalate with me because I do the preparation beforehand. Of course the logistics sometimes prevent this, but it's always a good thing to keep in mind for when time and space are in your favor.