Webinar showing message - "Webinar full" - but only 161 participants in the webinar

Hello, I have a Webinar 500 participants subscription. This morning during a Webinar session, there are about 160 participans on-line, but some clients tell me that they are unable to join the Webinar and their devices are showing a message like "Webinar full" when trying to join the Webinar. Why Zoom is showing this message if my subscription is "Webinar 500 participants"?