I've come across similar issue, Google will display rich snippets if they are correctly implemented using "site:" but they wont display it on normal results if they are not sure about the quality.
John Muller discussed about this in his Office Hangouts
Usually if you do a site: query for a site we will show the rich
snippets if its implemented technically correct so thats a good way to
kind of double check we can pick up the mark up properly and for not
showing it in normal search results then thats really a sign that we
are essentially not sure about the quality of the website
I can see you have only 1 review on your product that may be an indication for Google not to show the rich snippet.
I would advice you to try and get some more reviews. Also test products with more reviews to figure out if Google is not showing your rich snippet because of the site quality or because you dont have enough reviews.
The main takeaway here is that you have correctly implemented the rich snippets but Google is hesitating to show them because:
- You need more reviews
- You need to improve content/site quality
Try to increase your sites trust flow is one of the most important metrics when you are trying to win rich snippet positions.