Thanks for posting in the community.
Based on the doc here, Shotgun will display launchers for all supported software packages that it finds in standard application directories. Could you ensure you installed Maya 2020 in the standard application directories, the same as other versions which appear on the Software entity.
It’s okay if your DCCs are not in the standard application directories, but you probably need to manually fill in the
Linux/Mac/Windows Path in order for Shotgun to find it. And a reminder that:
When you specify a value for any of Linux Path, Mac Path, or Windows Path on a Software entity,
that entity will shift to Manual mode. Unlike auto-detect mode, where the software would show up
in Desktop when a path field is empty, in manual mode, a software package will only show up on a
given operating system if a path is specified for it and the file exists at the specified path.