Please follow the below url it is explained related to patch download issue on client machines.
- https://www.ajtek.ca/wsus/client-machines-not-reporting-to-wsus-properly/
- https://social.technet.microsoft.com/Forums/en-US/e57ef3c7-0e9d-46e6-9584-3df74d2d1cbf/wsus-updates-were-approved-but-those-were-not-downloaded?forum=winserverwsus
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