One of the most common reasons for a selected item not displaying in a Storefront is the item not being flagged as 'In Stock'.
Go to your list of Storefront Items:
From there, you'll see a list of your Store Items. Disregard the 'Status' on that listing, as we've seen instances of it being out-of-sync with the actual stock status setting of the item (something we're correcting on our end).
For any item that's not showing up on the Storefront, even when you've selected it to display, drill-down into the details of the item and verify the 'Status' column is set to 'In Stock':