Missing source element: foo

atom:source elements SHOULD include at least the required feed-level Metadata elements (atom:id, atom:title, and atom:updated) in the atom:source element.

This may just be a typo. Element names are case-sensitive; make sure you're using the right case.

Otherwise, insert the value of these elements as they appear in the source feed. Additionally, consider dong the same for any child atom:author, atom:contributor, atom:rights, and atom:category elements.