Hello @Netelixir,

Welcome to the Klaviyo Community!

At this time, the Klaviyo back in stock function is built to only work with any of the basic free Shopify themes listed in the Back in Stock Styling Code Snippets for Free Shopify Themes Help Center article seamlessly. This doesn't mean you wouldn't be able to implement this for third-party or custom themes such as the Symmetry Shopify theme.

These third-party and custom theme developers are not required to follow the same guidelines that allow Klaviyo's back-in-stock feature to function correctly. Although Klaviyo is unable to offer a custom solution for these custom or third party themes, the theme developer will oftentimes be able to help with this implementation or provide guidance on a solution. Alternatively, you can also reach out to one of our many wonderful partners from an extensive network of agency partners for further assistance in customizing the Klaviyo back in stock code or your website’s theme code for this feature to work appropriately. 

If you end up working with a developer, you also have the option to build a fully customized solution. Klaviyo’s Back In Stock feature works by sending an event metric into Klaviyo which triggers a flow. You just need to send the Variant ID, Product ID, and email address. All of this can be custom-built using our open API to work with your website. You can learn more about this from the Shopify Back in Stock for Developers guide we offer.

Otherwise, there are other back-in-stock solutions supported through Shopify that you can find through their apps section. Here is a link to their site: https://apps.shopify.com/search?q=back+in+stock

Having the “Continue selling when out of stock” setting enabled within your Shopify backend is also another common cause for why the “notify me when available” button not displaying. I’ve detailed this previously in the Back in Stock Flow - "Continue selling when out of stock" Community post. Klaviyo’s back in stock works by detecting your catalog’s inventory levels and triggers on your site when the product/variant’s inventory level is zero. If you sell made to order products or digital goods whose inventory is not being tracked, Klaviyo’s back in stock model would recognize this as out of stock and appear on your website. 

I hope this helps!

David