We find that we have to tell a lot of customers that they can set up restock notifications, as the current presentation of Out of stock sizes does not encourage clicks that then highlight to them that they can set up a restock alert. Restock alerts increase our immediate sales massively because as soon as the product lands in stock, they are all snapped up again. Particularly important whilst stock is intermittent, yet in high demand.
On the product page, when a size is "out of stock" it is currently greyed out and therefore customers will be less inclined to click on it to trigger the "notification" CTA.
I'm not entirely sure what my suggestion for alternatives would be at this stage but I'm mulling it over.

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