Well, from your other thread it sounds like she wasn't into it -- she was doing it for you, which was sweet -- way more accommodating than my last gf, for instance, where she wouldn't even let me touch her feet (although she wore these really sexy socks, sort of rainbow-stripey black things with individual toesies on them -- a guy sees something like that he gets ... expectations, so I'm calling that mixed messages).
a) You have all of these wonderful memories. 2) You got to tickle a beautiful woman for an extended period of time, suggesting that is something that happens and could happen again. c) If your lee is losing sensitivity in any given place, first step is to talk to her and find out if she's on any new medications or if she's changed her exercise routine, new shoes, is she going barefoot more often, or if there's anything else that might be going on.
Body chemistry can affect brain chemistry, after all, which provides the secondary neural response to tickling, and if you wear out those pathways ... yes, you mentioned changing up technique and her levels of anticipation, but that clearly wasn't working so it's possible she indeed became inured to your ministrations.
But feet are little. The body is long and lithe and full of nerve endings and nodes and
so many more ticklish spots.
And if she's actually not into it herself and is only doing it for you, consider dialing the sessions back to a frequency that doesn't WEAR HER OUT!