@wernstrom sorry that the Back in Stock for BigCommerce is not working for your store.  It would be hard to give you any suggestions without knowing which BigCommerce theme you are using as well as sharing an example product page where variants are unavailable.  You may not want to disclose this in a public forum so best I can offer you is that you email support as indicated here

Hi @klaviyojad thanks for your response. Unfortunately, it seems like I lost the Klaviyo support lottery and got a response about Shopify themes, despite mentioning I am using BigCommerce in the first sentence of my message (and providing a suitable product URL). This seems like a pattern when it comes to BigCommerce and Klaviyo.

@wernstrom apologies for that.  We have many people working in email support and they should eventually route you to someone with more expertise on BigCommerce BIS matters.  Appreciate your patience.

@klaviyojad I understand that, but for them to completely miss the point on the first response is not great. Klaviyo needs to be more upfront about what does/doesn't work with various platforms rather than just “check out our awesome feature...oh yeah, but it doesn’t work on X”.

Wait, so does it work or not on Bigcommerce?  The support articles suggest it should work, but I’ve run into the same problem as you.  I’m not getting the Notify Me button appearing on out of stock products.

It does work. IIRC the issue I had was related to some sku bundle components that had been changed in BigCommerce. 

They provided me with some new code, that shared to fix it at first glance.

But after looking at it again while checking the flow I've found it's inconsistent. The button correctly appears on some out of stock products, correctly doesn't appear on some in stock products. However, last night I noticed an in stock product that has both the notify me when available and add to cart button and an out of stock product the button isn't appearing on.

I sent this to sorry last night and I'm waiting for a reply.