Swatches aren't showing

Work through the checks in order - they’re sorted by how often each one turns out to be the cause.

Three checks before anything else

Is the app embed on? Swatches only render when the matching embed is enabled in your theme editor: product page widget for product pages, collection page widget for collection pages. They’re separate toggles - having one on doesn’t turn on the other. Check under Online Store > Themes > Customize > App embeds.
Did you recently publish a new theme? App embeds are per-theme. A newly published theme - a redesign, a fresh install, even a copy of your old theme - starts with the embeds off, so swatches disappear the moment it goes live. Open the theme editor for the new theme and turn the embeds back on under App embeds. Planning a theme switch? You can enable the embeds on the unpublished theme ahead of time - the setting carries over when you publish. If we’d configured custom placement for your old theme, the new one may need the same treatment - contact support and we’ll set it up.
Did you just set this up? Swatch data syncs to your storefront through Shopify metafields, which takes a few minutes. If you created the group or enabled the feature moments ago, wait a few minutes and reload. See changes aren’t appearing for how syncing works.

Is the product in a group? A product only shows group swatches when it belongs to a group. Open Product groups in the app and search for the product.
Where is the group set to display? Each group has a Displayed on setting - product pages, collection pages, or both. If swatches show in one place but not the other, this is usually why.
Is the product hidden by a rule? With Hide inactive products on (Settings > Product groups, Rules card), draft, archived, and unpublished products drop out of swatches by design. And with Hide groups with only 1 swatch on, a group that boils down to a single visible swatch is suppressed entirely.
Is the group synced? Open the group in the app and check its status. A group stuck on syncing for more than a few minutes usually means a Shopify hiccup - it recovers on its own, but contact support if it doesn’t.

Is the feature on? On the Variant swatches page, check the Status card. With Only selected products, swatches render only on products in your list - a product missing from the list shows the default dropdowns.
Your theme’s variant picker must be on the page. Variant swatches work by restyling and driving your theme’s own variant picker. If the product template has no variant picker - the block was removed in the theme editor, or a custom section replaced it with its own markup - the app has nothing to drive, so it hides itself and leaves the page as-is. Check your product template in the theme editor and make sure the Variant picker block is present.
Single-value options may be hidden on purpose. The Hide single value swatches setting (Appearance > Variant swatches) hides an option that only has one value. A one-variant product shows nothing - that’s the setting working.
Check the option’s swatch style. An option only renders as swatches once it has a style assigned on the Variant swatches page, and a Color or image option needs its values linked to swatches.

Collection swatches have their own embed and depend on your theme’s product-card markup, so they have extra failure points:

The collection embed is separate. Enable Collection swatches under App embeds - the product embed doesn’t cover collection pages.
Using a filter or search app? Apps like Boost and Globo re-render the product grid when shoppers filter, which drops the swatches. See Integrations for the wiring.
Is the option hidden from cards? In Settings > Product groups, the Collection display card controls which option names appear on collection cards. An unchecked option shows on product pages but not on cards.
Non-standard theme? If the app can’t find a spot on your product cards, add the container by hand with the manual snippet, or contact support and we’ll configure the selectors for your theme.

Add ?pl_debug=1 to the page URL and open the browser console - the debug output shows what the widget detected and why it skipped rendering.