Each day we'll pull the latest store data for your selected products and categories to advertise. This means we also check the stock status for each product being advertised. If your product is out of stock, it will not be eligible to advertise. As soon as the product is back in stock, the status will be updated within 24 hours max automatically, and be eligible to start showing that product ad again.
If you visit the Google Shopping --> Manage section and click the pencil icon, you'll visit the Edit Active Campaign page. You can then go to your selected products section, and click Refresh My Store. This will get the latest store data at that time, allowing you to sync changes sooner if you'd like.