The main reasons:

1. It’s a new collection and it hasn’t been indexed by Searchanise yet. Searchanise doesn’t index collections automatically, so after creating one you need to click the Force re-indexation button in the sidebar.
2. The Filters on collections feature was disabled, but, in case you had set up Navigating through embedding HTML code into the Shopify template, you didn’t return the Shopify code that shows collections.
3. Your Searchanise subscription was canceled, but, in case you had set up Navigating through embedding HTML code into the Shopify template, you didn’t return the Shopify code that shows collections.
4. You set up a link to a collection using the Filter collection with tags setting in the Shopify navigation menu. However, the app’s filters based on tags or filter values corresponding to tags in the menu are hidden. In this case, you should:
   1. Go to the Smart Search Bar & Filters control panel > Filters > Filter list section.
   2. Check that the status of the filter by tags is Active. If it is Hidden, edit it to Active.
   3. Check that the status of the filter value used for filtering the collection in the menu is Active. To do that, click the Manage filter link next to the filter by tags. If it is Hidden, edit it to Active.