I'm wondering if you have all software sources now which are used in Zorin 17.
I found here a thread which shows the sources files:
Edit: Oops, I think I'm on the wrong track... Maybe it's a problem with policykit so that you don't get a prompt to enter the password for sudo rights. Then those apps wouldn't work, only the terminal.
What you can also check is if you have set a metered connection in your network settings. If so, disable it for testing. It can prevent updating packages.