I tried everything, but Ubuntu simply doesn't show any updates. How do I resolve this?
You have to click 'Check' in Update manager And the enter your password .
try opening the terminal and typing
sudo apt-get update after it finishes run
sudo apt-get upgrade If you get the following message
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Then there are no newer versions of your software available, and you should check at a later time.