Unfortunately this is not something that is 100% "fixed" once nopCommerce is configured properly. Google determines wheter or not to show these sitelinks, based on relevancy for the user in relation to the searchterm/query.
The best way to go now is to check the Structured Data of the template this store is using. Google explains a bit here.
There is a so called Structured Data testing tool which tells you if the structured data is integrated in the template correctly.
The easier Google can determine which content the store has to offer, the higher the chances are that sitelinks or other forms of "rich results" are shown.