Magento 2 upgrade schema may not work due to version mismatches, broken upgrade scripts, or missing module version updates in the module.xml file. This often leads to schema not applying correctly during the php bin/magento setup: upgrade command.
Using a Magento 2 Upgrade Service helps identify issues like:
Incorrect logic in UpgradeSchema.php
Skipped version checks
Incomplete setup scripts
Module conflicts or database errors
Such services can assist with proper versioning, script structure, and testing the upgrade process step by step.