The first thing to check are the permissions on the drive where the Windows Server Update Services downloads are stored. As an automated service, this is something that your system administrator should be aware of and able to amend as necessary.
Downloads are stored in the \wsus\WsusContent folder on your operating system hard drive, along with the various subdirectories – the Network Service account (which is used for managing downloads and their distribution) should have Modify rights to these locations.
Amendments to the folder permissions can be made by accessing the Properties of the folder, using the Security tab. Note that to make all child folders carry the same access properties, you will need to choose the option Inherit from parent the permission entries that apply to child objects.
By awarding the correct privileges to these folders and their files, the Network Service account can now correctly manage the downloads, which should bring to an end any issues you have had in completing the downloads.