You can check if you have a non-standard update source:
defaults read /Library/Preferences/com.apple.SoftwareUpdate CatalogURL
If CatalogURL does not exist, that's OK and the standard situation to pull from the non-beta updates. To force a beta source, you write this key (as root if needed with
sudo /usr/sbin/softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.15seed-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz
About This Mac >
Software Update. Wait a moment. An item for
macOS Catalina Beta appears.
You can request an update as well from the command line just as you set the default:
Change the lower case L to a lower case I above and prepend
sudo to download and install all updates.