I guess, you will have to open a terminal, and use the command line :
$ sudo apt-get update
.. I.e. there is no updating of the package listings in Muon, AFAIK.
That's probably why dozens of packages sometimes are missing.
Suggest : Install a better 'Package Installer' : synaptic
→ $ sudo apt-get install synaptic
.. "synaptic" 1) has a Refresh button 2) All packages are present.
But : You will always have to hit the 'Refresh button' before installs,
to insure that all updated packages and dependencies are available.
P.S. : It's only the 'Muon Software Center' that often fails,
the "Muon Updater" is always working OK.
Last edited by knudfl; 09-30-2014 at 05:13 PM.