A product in my bundle is marked as out of stock, even when it is in stock. What can I do?
It could be that you modified product variants for this product, which caused Shopify to change the variant ids.
Now, the bundle is configured with old variant ids which don't exist anymore. This is why the product is marked as being out of stock.

To resolve this issue, edit your bundle, select the products again, add them to the bundle and save the bundle.
This will update variant ids and the product will no longer be marked as being out of stock.