In the background, the YARPP plugin works, its algorithms determine the results.
The essence of the operation is to weigh the individual elements of similarity, and there is a match threshold that determines how similar elements are included in the results. If there are not enough result, this match threshold should be reduced.
In the Settings-> YARPP menu, you’ll find a Match Limit field, the default value is 5, try to reduce it until the number of hits you see appears (e.g. set it to 2).
OR if you would like to specify the match threshold between product and product only, go to the “Products Page” tab on the WP Related Items Settings (Settings -> WP Related Items) page, and set a lower match threshold in the “Products” row.
You can find more information on the Documentation/FAQ page of WP Related Posts and Products.
See “What happened if related items do not appear?” question.