This would either be because it is a bundled item (Hydra does not list Walmart Bundled Items) or the item is being sold by a 3rd Party Marketplace Seller and not Walmart. Hydra now has ability to list 3rd party marketplace sellers. All you have to do is add #skuopt:all to the end of the supplier url.
Let's look at this example:
This item is being sold by BikeWagon on Walmart's platform but not sold by Walmart.
You see under Add to Registry/Add to List, Walmart shows the item is being sold by Bikewagon and not Walmart.
If you try to list the regular Url with Hydra, you will receive the following error: There was a problem retrieving a product from the supplied url
To solve this problem, add #skuopt:all to the end of the supplier URL. So the original url would go from