Google in July announced that product offers in US now require unique product identifiers. "Google have begun to make these changes in the US. Product offers in Electronics, Books, and Media categories that're submitted without Unique Product Identifiers mayn't be listed, or may appear in a lower position in the results," announced Google. We recommend checking the Data Quality tab in your Merchant Center account to see examples of products which may have missing or incorrect identifiers."
More Info: Help Center