Software Center have some trouble to download large update.
In deed after few download/install, the download seem freeze/very long.
For exemple: Software Center said "download Libreoffice 1 MB/s" during 20~30 min with any progress.
If you check the network load with the ressource monitor you will see any download.
How to reproduce:
I have just made a fresh install with the lastest iso. It's preaty old now
I after the first boor made all the available updates and watch the network load.
How to fix it:
Use "eopkg upgrade"