Are the values in the same case in both sources? Some correlation rules are case-sensitive. The only way I’ve gotten around that is to export the source via SP Config, update the “ignoreCase” flag to true, then re-import.
There’s also the “Identity Fusion Connector” which may make sense to try to implement if you’re not already too far down the path of implementation to change course on something so foundational. New Capability: Fusion Connector for Identity Security Cloud! - Announcements / Product News - SailPoint Developer Community