Common failure points
Identifiers usually go missing when catalog data lives in one system, the storefront in another, and the structured data layer is left to guess from visible text.
BrandUse the actual manufacturer or brand entity, not a generic site name.
SKUKeep the SKU stable across feed, product page, and markup.
GTINOnly publish a GTIN that belongs to the exact item.
MPNUse MPN where the catalog actually maintains one.
How ProductProof frames the fix
The report separates missing identifiers from mismatched identifiers, because the remediation path is different. That is the kind of detail ProductProof packages into a Store Audit and Fix Pack.