Hi,
Sorry for the issues you are having. I can help troubleshoot this with you.
First if you had coding to move the ticket box for eventbrite in your theme that has changed and please check the following guide to update that.
Moving the Eventbrite Tickets Box
Second, can you please confirm you are using Eventbrite Tickets 3.9.6 and that you have followed the configuration steps outlined here:
https://theeventscalendar.com/knowledgebase/configuring-eb-tickets/
Then can you go to an event that is suppose to show the tickets box and confirm this setting in the admin for that event is set to yes:
Display tickets on event page?
Even if it is set to yes can you please set it to no, then update, and then set back to yes and update again and see if it shows.
If that does not work please post a link to the event that is having issues and we can go from there.
Thanks