Why custom buttons cannot be displayed on the Events Management detail page on Veeva CRM Online？
One possible reason is that the custom button is not added to the Visible_Buttons_vod field of the EM_Event_Layout_vod record being used.
The Events Management admin can identify the EM_Event_Layout_vod record used for the end-user by using the Page Layout Organizer function, and check whether the Visible_Buttons_vod field is populated with the custom button.
Page Layout Organizer:
CRM Help Documentation: