There could be many reasons why your announcement banner isn't showing up. Here are a few of the common ones.
1. Your banner is not properly installed on your website.
In order to display banners on your website, you'll need to install the script tag on your website. The script tag should be installed within the <head/> tag of your website's HTML code. If you installed and authenticated using the Shopify app store, the script tag will be automatically installed onto your store's website. Please refer to the installation documentation for more details.
2. Your banner is currently in "DRAFT" mode.
In order for your banner to display, it will need to be in the "ACTIVE" mode. Please refer to the "Enable and Display Your Announcement Banner" section of the How to create your announcement banner article.
4. You have an error within your custom HTML banner.
If you are using custom HTML within your banner, please check for syntax errors. This will only happen if you use the "Code Editor" within the banner designer. Reach out to firstname.lastname@example.org if you need help troubleshooting your custom HTML.