my experience
Interesting replies, all.
I'll elaborate on my experience: when my then future-wife and I started to become more than friends, I tried tickling her to no avail - and I tried everywhere. She said that she was only mildly ticklish in one spot (tops of feet), but could generally control it. It was pretty disappointing, but such is life. After we'd been together for a little while, occasionally she would react a little more strongly, and in a few different places - not great, but enough to keep me trying! Any time she was ticklish, she seemed surprised by it.
After we'd been together much longer, occasionally I would get a few strong reactions, depending on the circumstances (e.g., if I was stroking her very lightly, say during pre-foreplay (if there is such a thing!), and "accidentally" brushed a sensitive spot, more of these sensitive spots would appear, and then very rarely she would become ticklish over much of her body, and react very strongly, and I could tickle her productively, but as soon as she got a minute to catch her breath, she could shut it off again. Now that's frustrating!
Overall, I'd say she's not ticklish at all about 50% of the time, mildly ticklish 45% of the time, and extremely ticklish 5% of the time. But there doesn't seem to be any pattern - when those 5% of occasions arise, it seems completely random to me. Although frustrating, it's actually pretty interesting from a psychological standpoint, and I'd mention (as casually as possible) that it was pretty weird how she could go from 0 to ticklish and then back to 0 again, and ask her (again pretty casually) about her past ticklishness. She said she'd never been very ticklish as a child, but that as a teenager she'd be mildly ticklish in "all the normal places" when she'd first start dating a guy, but then it'd disappear after a while.
During those rare times when she is very ticklish, she says that she has no idea why she is so ticklish then (almost apologetically! I think she knows that I like tickling her, but she doesn't know that i LIKE tickling her, if you know what I mean!). But it seems, that buried deep underneath a non-ticklish exterior is some latent ticklishness that shows itself every once in a while. Again, frustrating but interesting at the same time. That's my long example of "variable ticklishness". Thanks for your patience!