You’ll notice that the Check for Updates button goes away in Windows 10 or 2016 once an update is pending. To manually retry seeking for updates, forcing Windows to check for updates again (e.g. because you’re hoping to get an update that fixes a broken update), run this from an administrative command line prompt:
usoclient StartScan
So that’s how you kick off a Windows Update scan from the command line.
Examining the Task Scheduler tasks under Microsoft > Windows > UpdateOrchestrator is instructive. We have
usoclient StartScan
usoclient StartDownload
usoclient StartInstall
usoclient RefreshSettings
usoclient ResumeUpdate
Not sure what all of those do, but if you have the Updates pane open under Settings when you run a StartScan, you’ll see it re-scan for updates.