Hi @Adrian Joel,
Welcome to the Community! We’re glad you’re here!
Yes, unfortunately, for stores which have a custom theme, Klaviyo's pre-built back in stock flow and syntax are designed to work with free Shopify themes, but the coding we use for custom themes is not supported. However, you can still receive the personalized help you need! We have developer documentation for implementing back in stock code in our developer's portal. This does require you to build a custom solution, but also provides total control over the user experience.
While I can’t work with you to find a personalized solution to your BIS feature issue, I can share that in my experience, I have seen this happen to website when a product catalog isn’t published, thus seemingly like all product inventory is zero.
I would recommend reaching out to one of our awesome partners in our partner directory to help you find a custom solution!
Thanks for your participation in the Community!
-Taylor