Hi!
First, you need to check log on client side to determine, why this client is unable to download updated/new "Windows assessment scan" package from Package Server.
- Logs are here: C:\ProgramData\Symantec\Symantec Agent\Logs
Or make mouse double click on Symantec Agent systray icon and open "Software Delivery" tab, then double click on "Windows assessment scan" package and check "Download History" to determine whether there is source to download this package or not.
Probably that your Package Server doesn't have this latest package downloaded/published yet to NS server, but client is trying to download it already. Try to refresh policy on Package Server and then check, what will do a client PC.