Windows 10 Update 1809 not offered even if I manually search for it

I am operating two different PCs both running Windows 10 Professional x64 Build 1803. I know that Microsoft has delayed the update to 1809 because of some problems, but I have also read that the update is supposed to be available now, and that installation can be forced if a user checks for new updates manually in System Control -> Windows Update. However, I have tried clicking the "Check Updates" button in Windows Update, and get the message that my system is up to date.

So my question is: why does Microsoft not offer that update? Is it possible to somehow see the reasons why an update is not available even if cecked manually?

Update (as requested): both of my PCs are on the Semi Anual Targeted stream, so they definitely should get the update. Is there any log file that informs about reasons why no update is applied, possibly incompatible hardware drivers or other software installed that Microsoft is aware of?

Update 2: I also know that I can manually install the update, or reinstall a fresh build 1809, but I want to know if there is a logfile or other mechanisms to examine this in detail.

I believe this blog post by Microsoft may "answer" the question:

https://blogs.windows.com/windowsexperience/2018/10/02/how-to-get-the-windows-10-october-2018-update/

See the fourth and fifth sentences in the paragraph following the Want the Windows 10 October 2018 Update today? Start by manually checking for updates heading:

We are also streamlining the ability for users who seek to manually check for updates by limiting this to devices with no known key blocking issues, based on our ML model. If we detect that your device has a compatibility issue, we will not install the update until that issue is resolved, even if you “Check for updates.”

Thus, it seems that the automated determinations governing this behavior are opaque and possibly take place entirely on the server side, based on miscellaneous system information uploaded by Windows Update.

I know this does not provide an answer to OP, but I would have liked to know this, as it helped me with my question which was exactly this question's headline:

Today it's March 4th and I was still on 1803. I have this update on my home computer and this one is my work computer. But no update. It made no sense to me. My boss had the update, but not my coworkers and me.

However, when I changed from "Semi Annually" to "Semi Annually (Targeted)", the update became available instantly and it is installing as I write this. And, as expected, my boss is using the targeted channel. By the way, the channel selection is found within "Advanced Settings" under Windows Update.

Thanks to Ramhound for the suggestion.

If the update is not being offered automatically you can get it via the "Update Assistant" tool, as per https://support.microsoft.com/en-us/help/4028685/windows-10-get-the-update