Several of our Windows 10 clients on 1903 will not update to 1909. Several has updated using "Feature Update via Windows 10, version 1909 Enablement Package - Applies to: Windows 10, version 1903, all editions" KB4517245 - but others are reporting that it is Not Applicable. Similarly, the other Feature Update packages for 1909 are showing Not Applicable. All the computers were created from the same image so it makes no sense.
The WSUS is up to date as are the client side components.