When you have reviews linked to product groups (particularly if it has many variants), there is a limit to the number of product variants a review can be linked to on your Google Shopping page.
Currently, it is set at 150 reviews, after this, Google will reject a review if it has too many products and/or variants linked to it.
What To Do To Ensure Your Reviews Aren't Rejected
Keep your product groups small
Ensure your product groups (reviews) are within the 150 limit