My connection is fine but when I open the software center, the install button is grey. It says I'm not connected.
I have tried the following:
*Reinstalling, updating and upgrading
*Changing server
*Rebooting computer (yes)
(update screenshot) https://s29.postimg.org/4jzr6zaef/sample.png
This is years after the fact, but I just came across this same issue, and resolved it with a tip found elsewhere.
In my case, it was resolved by going to terminal and using:
sudo apt purge update-manager update-manager-core && sudo apt install update-manager
It appears that there have been some updates to Update Manager, and this changed it's network checking behavior.
The only change log entry I could locate for networking was from late 2010 though.
Try doing update via terminal first sudo apt-get update then upgrade everything sudo apt-get upgrade ,then run software-center and try again.
