If an item or size is sold out, you see that on the product page. There you can request a stock notification: enter your email address and you automatically get a message as soon as the item is available again.
Whether and when an item comes back, we often only know shortly beforehand. Our customer service has no other information about this than what is on the product page. A stock notification is therefore the quickest way to find out.
Some items do not come back; if an item is no longer in the webshop, it is out of the collection.