Problem
If you enabled third-party repositories during a fresh system install, some software from those repositories might not be shown in GNOME Software initially (there are a couple of rare race conditions - cases where operations happen in just the wrong order - that can cause this to happen occasionally).
Related Issues
Bugzilla report: #2016510
Workarounds
The best way to force GNOME Software to refresh all repositories and re-populate the search results is to go to its Updates tab and click the refresh arrow in the top left corner. After this is complete, all available applications should show up in the search results.