If you add products to a "Subscription Group" that is out of stock and use Shopify inventory tracking, the Recurring Orders widget will not appear on the product page for those products.
If you track inventory, make sure you always have at least one item in stock. Alternatively, you can turn off inventory tracking for your products.
If a product is or becomes "Sold Out", any subscriptions containing those products are unable to process and will be delayed.
Visit Managing your inventory for more details.