Something seems to be wrong with the updater tool in Zorin 18. Some users no longer receive notifications about new updates. As a workaround, it may help to add the software updater to autostart aa shown in this thread:
But when the software updater doesn't find updates at all this workaround wouldn't help. Then maybe sudo apt update && sudo apt upgrade (if you use flatpaks and snaps also flatpak update and sudo snap refresh) added to autostart could help.
Hopefully, the developers will be able to fix the problem soon.