One reason for missing keys can be due to how tags are sourced for Flex Reports.
Tags within Flex Reports are currently only sourced from within the CUR. This means that each tag key you wish to be present within FlexReports will need to be activated as cost allocation tags within AWS.
Please review your activated tags as described in the AWS article below and reach out to support if a key is missing and is activated:
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/cost-alloc-tags.html