You might notice that Google Product Reviews XML feed does not include all the product reviews that exist on your site. It is most likely caused by the fact that some of your product reviews are "rating-only" (i.e., customers did not write any comment with their reviews). The plugin excludes rating-only reviews from the Product Reviews XML feed because Google does not accept reviews without comments. If you try to submit rating-only reviews to Google, they will be rejected by Google and flagged as errors.
In some countries, Google might even reject reviews that have very short comments like "OK" or "Good". If this happened to you, you can use the setting "Minimum Length of Reviews" (Google > Reviews) as shown below. It will enable you to exclude very short reviews from the XML Product Feed.
We hope that this solution was helpful. If you have any questions about the plugin, please don't hesitate to post them on our free support forum or submit a ticket for our support team (available for customers with Pro licenses).