Hey @collegis_education! The primary difference between the two tools is that the Rich Results Test shows what types of Google results can be generated from your markup, whereas the schema markup validator offers generic schema validation. So the markup validator may be showing that there are no issues with the way your schema is written syntactically, but Google may still have an issue generating a particular type of search result based on that schema.

Be sure to take a close look at the errors the Rich Results Test is throwing, as some are more like warnings - not every aspect of the schema is necessary in order for Google to generate a rich result, but obviously if there's a particular piece of the markup that's missing (i.e. In-Stock status), it won't be included in that result.