There are several rules that are common among Google Shopping users. These rules are specific to Google Shopping, for common rules see this article.
Fixing insufficient identifiers
Google Shopping uses several identifiers: in Europe and North America these are usually ID, MPN and GTIN/EAN. At least two of those need to be provided. It is not uncommon for channels to not always have at least 2 out of 3 identifiers available. In order to send products that do not have enough identifiers, the field 'g:identifier_exists' needs to be set to 'FALSE'.
The 'g:identifier_exists' field is not one that is commonly included in product feeds, so you'll usually need to create it first. See this article on how to add a field. Call this new field 'identifier_exists' so that you can easily identify it. Then, in the Rules for your Google Shopping channel, create a new rule where you set 'identifier_exists' to 'FALSE'.
Be sure to map your new field to 'g:identifier_exists' and update the feed to see the results.