Usually, whenever you install Ubuntu and any of its official flavors AND system based on Ubuntu and using Ubiquity (Ubuntu Installer), there is a steps during the installation where you choose "Where are you". That is the steps which will be responsible to choose which server that your system will use 'after' the installation to download from any package from the repertoires. There is main server and there are mirrors. Whenever you have issues, the first thing you may think about is to change the server to the main.
For me, I always do 'Select Best Server' - as explained on the above link.
Remember, after selecting different server, you need to refresh your list by:
sudo apt-get update
Hope that helps :)
--
Remember: "All of us are smarter than any one of us."
Best Regards,
