If you have a Stencil theme, you may experience the following problem.
The Instant search widget is loaded only after the whole page is loaded. That is why sometimes the widget is not displayed right after the page is opened.
As a workaround you can try to add the instant search to your store through the Theme editor.
1. Go to BigCommerce admin panel -> Dashboard -> Storefront -> MyThemes.
2. Click Advanced in the Current theme block. A list of actions for the theme will appear.
If there is an Edit theme files option in the list, choose it, click Edit theme files in the appeared dialog box and go to the point 6 of this instruction.
If there is no Edit theme files option in the list, choose the Make a Copy option and follow the next steps.
3. Specify a theme name (you can leave the default one), click Save a copy and wait until the new theme is loaded.
4. Go to the appeared theme in the My themes section, click "..." and Apply.
Then choose the necessary style and click Continue.
5. Click Advanced in the Current theme section of My themes. Choose the Edit theme files option and click Edit theme files in the appeared dialog box.
6. You’ll be redirected to the Theme Editor. Open the templates/layout/base.html file there and insert the following Searchanise widget code before the </head> tag:
Replace [api_key] with the API Key of the Searchanise engine for your subscription. You can find it in the Searchanise dashboard. ( BigСommerce admin panel -> Dashboard -> Apps -> Searchanise).
7. Click Save & Apply file and confirm the action by clicking Apply in the appeared dialog box.
Done! Not so difficult, isn't it?