There are several possible scenarios where a user who is registered in a live session event (initiated with a Zoom account) won't be able to see the "Join" button. Specifically:
>Unconfirmed email address
When you hit on the "Initiate Event" button, the event is being automatically created on the Zoom side and all of the currently registered users are being validated from Zoom, using their emails.
The email is the only unique identifier that Schoox and Zoom have in common. So, all of the users who will be added as registrants should have a confirmed email tied to their account.
There is a setting in Zoom which is called "Only authenticated users can join meetings". You need to check if this setting is turned Off because, during the Initiation process, Zoom validates the users (based on several settings like this one) and replies back to Schoox with an API call. Depending on this response, the "Join" event button is shown/not shown to our users.
>Event not started through Schoox
After the event-initiation, you are ready to Start your Event! Just click on the relevant button (Start Event) and you will be redirected to your Zoom account. Please note that if you do not Start your event through Schoox, the users won't be able to see the "Join" Event button when accessing the Preview page of the event. All of the actions should be completed through Schoox.