Widget not showing on Shopify
On Shopify the widget is usually missing because the Custom Liquid block was added to a template the page does not use, or the change was previewed rather than saved..
Platform-specific problems
Four platforms have failure modes of their own. Everything else uses the common pages.
What is here
Only four platforms get their own page, because only four have failure modes that differ from the generic ones: Shopify app blocks and templates, WordPress caching and security plugins, Wix iframes that crop rather than scroll, and Squarespace code blocks that do not render in the editor.
Everything else is covered by common problems.
Browse
Every page in this section, with the rest linked from each one.
Widget not showing on WordPress
On WordPress the two usual causes are a caching plugin serving the old page and a security plugin stripping the script from the content..
Widget not showing on Wix
On Wix the widget usually does show — it is cut off.
Widget not showing on Squarespace
On Squarespace the usual cause is the editor preview — Code blocks do not always render there.
Why only four
Because only four have failure modes that differ from the generic ones. Adding a page per platform that repeats the same advice would pad the section and make the four that matter harder to find.
If your platform is not here, the common problems pages cover it — the causes are the same and only the menu paths differ.
The pattern across all four
Check the code is on the page being viewed, check the change was saved rather than previewed, clear the cache, then look at the console. In that order.