No screen-share button
Your browser may not support sharing a screen. This is common on phones and tablets, including when desktop-site mode is enabled. Try a supported desktop browser.
Sharing will not start
There can only be two screen shares in a room at once. If both places are in use, wait for someone to stop. Check your computer's screen-recording permission for the browser if it asks for one.
No sound
Stop sharing, pick a browser tab, and enable the audio option if the chooser offers it. Sound support varies by browser and share type.
Black movie or video
The site you are sharing may protect its video. Try a supported media-player video or content that permits sharing.
Sharing stopped by itself
A share ends after 2 hours. Start a new one to continue.