the Chat button is missing, in the zoom application on the PC. In mobile version I see it, and can use. But on PC, I can't see messages from other participants.
Without knowing a lot more about the specific situations, I'm thinking that in the meeting where you did not see the Chat button, the Chat was disabled. Sometimes buttons that should appear in the main menu bar are moved over into the "More" button's sub-menu. Always check there for any missing buttons.