Mac can't find new update

Suggest restarting in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

Then try the Software Update applet again.

Take special notice of it does work, as there maybe more than one update being offered. It could be offering an upDate to Big Sur to version 11.6.5 and in that same presentation offering the upGrade to Monterey 12.3.1

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - Software on the computer interfering with the download process including AntiVirus Software. Notation: Deactivating the AV Software may or may not work. The removal of such Software may allow the update / upgrade to function.

5 - Running behind a VPN which may increase Packet losses and corrupt the download

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

7 - Best Results are via Ethernet Cable to Router

Lots of people getting the update; the server is likely very busy. Be patient and keep trying. Our five Macs took most of one day for the update to be available for each.

Do keep us posted on the progress - if time allows