In certain cases change of download URL (by Vendor) for old Linux updates may not be detected by incremental import of meta data, so updates may fail to download.
sample updates (with outdated URL):
Symantec Management Platform 8.x
To update the URLs and fix the issue – uncheck Incremental import check box and perform Red Hat meta data import.
The Full Red Hat import does not need to run every time or often. Suggested once per month or on demand if there is an update with the issue.