If your Mac is not showing a new software update even though it's available, several solutions are available.
First, make sure that your Mac is compatible with the new software update.
You should also check if a new non-beta or beta update has come out, as your Mac may not receive beta updates.
In addition, the Software Update section under System Settings is not just for updating to a newer version of macOS. Updates to Command Line Tools and new symbols are also handled via System Settings.
If an update is available, switching back and forth between the “Software Update Available” option shown at the top of System Settings and General > Software Update can sometimes fix the issue.
If that doesn’t work, force quit System Settings and reopen it, restart your Mac, turn off VPN, change to a different network, or restart your Wi-Fi router.
It can also be useful to check if your Internet is working, as your Mac must have a working Wi-Fi, Ethernet, or Personal Hotspot connection for the new update to be visible.
If nothing above works, take a break and retry after a couple of hours, as Apple’s servers can be overwhelmed when a new major update is published.