Generally, once an item is listed as out of stock or removed from our store, we're usually not able to re-stock.
If an item's size/color is no longer available for selection in the dropdown options list while adding to cart, it usually means that that particular item's size/color have been sold out and will no longer be available anymore.
As we have thousands of items in our store, it is hard to keep track of which items will be restocked.