Gnome Software lies about "Software being up to date" after "checking for updates."
I've observed this bug for as long as I've used Gnome Software (about two years), and it's been present across distros and desktops (Fedora, Ubuntu, Gnome, KDE). You click the check for updates button, and - after a pretty lengthy refresh - it fails to display any additional updates. I've never the seen the update button actually fetch any new updates (that weren't already fetched by Gnome Software automatically). Why is this button even there then? It doesn't do anything.
I wonder if it has something to do with the cache.
(Side note - and this may or may not be related - in my experience, all the packagekit-based software stores completely break down periodically until you forcefully clear the cache from the terminal and kill packagekit. This is another long-term bug that is at least several years old which also affects all platforms, but much a more serious one since it makes system updates completely impossible). The issue is being documented here and here, albeit nobody knows what's going on.)
Sys info: Gnome Soft 3.30.6 Kubuntu 19.10 packagekit 1.1.12-5