In macOS 10.15 Catalina or newer, Ecamm Live needs special permission to share your screen. Without this permission, Screenshare mode will only display your Desktop picture, regardless of what you've chosen to share.
When activating Screenshare mode for the first time in Catalina, the app should have asked for this permission:
To change this permission at any time:
Open the Security & Privacy section of the System Preferences app.
Pick the Privacy tab.
Scroll down to the Screen Recording section in the left-hand sidebar.
Click the orange padlock to make changes, and enter your Mac's admin password when prompted.
Check the checkbox for Ecamm Live.