It is possible that the woman has simply "forgotten" her sensitivity. I have experienced this. I dated a mid-30's woman many years back. She was a recent divorcee. One night a few weeks into the dating we were sitting on my couch watching a movie. I had my arm around her and gave her a squeeze on the waist, asking her, "Ticklish?." Her response? Not even the vaguest flinch and she said she was not ticklish. I asked, "No? Not at all? Not anywhere?" "Nope," she replied. So I started probing her for spots. Belly, waist, ribs, feet, armpits, knees. Nothing. I did find a slight response, though in a spot on her ribs just under her armpit. Just the slightest.
I decided to work on that. Every time we were together, I would make pinchy fingers at her and go for that spot I had found. I found that over several weeks her response became greater and greater. One night I hit paydirt. She went nuts. Afterward, she lay on the couch panting, "OMG, I thought I wasn't ticklish!" The sensitive area spread slowly. By the time a month had gone by I could easily tickle her anywhere on her upper body - ribs, belly, waist, even armpits. At one point I was even able to squeeze her thigh and get a response. Then the ultimate - shortly before the end of the relationship I discovered that even her feet had become ticklish again.
