Some people just aren' ticklish anywhere, but they are in the minority generally speaking. With most people they're ticklish to varying degrees in various places at different times. Sometimes it's just a case of getting the time right, others it can be useful to judiciously apply something like baby oil to make the skin more slippery and thus more ticklish. Ice can be ticklishly agonising too.
Depending on how BDSM'ish you want to get, bastinado (coropral punishment on the soles of the feet with a cane, belt, riding crop, wooden spoon etc.) has enhanced the ticklishness of the lee's feet by about triple, both times I've used it!