So most of your clients see that they need this update, but didn't download it from your Wsus.
Can you look at C:\Windows\WindowsUpdate.log to see if any error appears.
Try to launch manually detection, in command window, type : WuAuClt /DetectNow
Can you look at C:\Windows\WindowsUpdate.log to see if any error appears.
Try to launch manually detection, in command window, type : WuAuClt /DetectNow