Captioning Doesn’t Work

I’m having some problems with captions in 2.6. Essentially, it appears that whatever processing is supposed to happen between the time I save and the time the post is published isn’t being done.

That is: in the visual editor the image and caption both appear to be there, centered as they’re supposed to be, and looking good. When I switch to “HTML” view, I see this where the image is:
[caption id="attachment_106" align="aligncenter" width="297" caption="This blog's archives page, shown with the Kaleidoscope theme, on August 6, 2008."]<a href="https://www.ikirudesign.com/wp-content/uploads/2008/08/kaleidoscope.png"><img class="size-medium wp-image-106" title="kaleidoscope" src="https://www.ikirudesign.com/wp-content/uploads/2008/08/kaleidoscope-297x300.png" alt="This blog's archives page, shown with the Kaleidoscope theme, on August 6, 2008." width="297" height="300" /></a>[/caption]

Having no experience with it, this seems like what I’d reasonably expect there (even if it’s not really HTML). The trouble is, when I actually look at the page, the image looks completely unstyled.

And viewing the source of the page shows that there’s no caption in the HTML. I get something like this:
<a href="https://www.ikirudesign.com/wp-content/uploads/2008/08/kaleidoscope.png"><img class="size-medium wp-image-106" title="kaleidoscope" src="https://www.ikirudesign.com/wp-content/uploads/2008/08/kaleidoscope-297x300.png" alt="This blog's archives page, shown with the Kaleidoscope theme, on August 6, 2008." width="297" height="300" /></a>

That was amazingly long-winded. And just to be clear, my theme has the necessary code. And I get the same result in the default theme.

Any help at all is appreciated.