So many factors affect ticklishness. Some spots are just more sensitive obviously, but you also have to look at how warm, how cold, how physically comfortable they are -- if they're all tensed up, that could be a factor, and a nice massage or even a warm bath first could make them more receptive.
Maybe the lee has a psychological block against tickling -- that is, he or she's feeling everything anyone else would, but their brain isn't processing it as would, say, some more responsive person's.
And then one's skills as a ler also play a role. If you're scraping away at a pair of feet with about the same reaction you'd get from a pile of towels, switch it up. Maybe tiny, wicked little movements right beneath the toes would wake up that laughter. Or deep-tissue stuff.
Or employ the fine art of neurological confusion -- one hand above the hip bone and the other in the armpit, say, or ribcage and inner thigh, or foot and belly. Information comes streaming in from all over the place and the brain can't just file it all away in a neat little box and so ... giggles.
Implements also have some potential. Feathers, brushes, electric toothbrushes ... I saw a classic video with Chad Warnes recently with one of those old-timey motorized massagers strapped to his hand -- hadn't seen that clip in years. Applied to a belly, one of those things could probably make Data ticklish.
All this being said, SadCuzNotTicklish will probably still be ... sad 😉, but in terms of ideas, one thing that I take great comfort in is how relatively similar we all are as human beings -- biologically, anyway. And with Shylock's observation from Shakespeare himself: If you tickle us, do we not laugh?
Most of us ... yes. 🙂