My app is not displayed in the placeholder “portal_tickets_details_sidebar”

Hi!
I’m developing my first custom app and I need to display an element in the end user front end of my custom theme.
I relied on the end-user-app example in version 3 and 2.3 and didn’t get anything to show up.
Just out of curiosity I applied another theme (Marina) to the portal and the element appeared.
Both themes have the {% snippet ticket_details %}.
What else should I check in the theme to fix it in my custom theme?
Thanks