Why Isn't a WooCommerce Widget Displaying in My Sidebar?

Why This Happens

Themes register their own specific widget areas (a main sidebar, a footer area, sometimes several more specialized ones), and a widget added to one area simply won't appear on pages that don't render that particular area — a widget placed in a "Shop Sidebar" area, for instance, won't show on the blog if the theme uses a separate "Blog Sidebar" for that page type. Modern block themes add another layer of complexity, since they may present widget areas differently or expect widgets to be placed via the Site Editor instead.

Common Causes

Widget added to the wrong sidebar/widget area for the specific page type where it's expected to show — check the theme's documentation for which areas correspond to which page types.
A block theme where the classic Widgets screen doesn't map directly to visible template areas the way it did with classic themes, requiring the widget to be placed through the Site Editor instead.
A caching plugin serving a stale, pre-widget-addition version of the page.
A widget-specific display condition (some widget plugins support conditional display rules) unintentionally excluding the page being checked.
The widget itself having no content to display under current conditions (an empty cart for a cart widget, for instance), which isn't a bug but can look identical to one.

How to Fix It

Confirm which widget area actually renders on the specific page type, checking the theme's documentation if unclear.
For a block theme, check whether widgets need to be added via the Site Editor instead of the classic Widgets screen.
Clear all caching after adding or moving a widget.
Check for any conditional display settings on the specific widget that might be excluding the page in question.

Classic themes: open Appearance → Widgets and note which area the widget is in. Then check that the page you're viewing, such as the shop, a product or the blog, actually uses that area.
Block themes: widgets are blocks placed in templates. Open the Site Editor and add the block to the template for the page type, such as the product catalog or single product template.
Look in the page source. Search for the widget's class name. If it is present but invisible, it's a CSS problem. If it's absent, it isn't being rendered, which means area, template or display conditions.
Check the cart widget's own option. It has a setting to hide itself when the cart is empty, which makes it look missing during testing.

Conditional display plugins

If a plugin adds show-or-hide rules to widgets, check them for the page you're testing. A rule written for logged-in users, or for specific page types, will quietly hide the widget everywhere else.

Need help getting theme widget areas working correctly? See WooCommerce fixes.