Please make sure when you add any content in the page builder block then you must click on save changes button at the bottom of the page.
However, based on past experience,this issue occurs because of server restrictions.
The ajax query which saves the page builder also requires to save the "HTML" associated with the page builder. However, some servers block the HTML and therefore the page builder never saves any content. reference :
https://wplms.io/support/forums/topic/page-builder-wp-editor-not-saving/page/3/
Consult your web host and ask them to allow admin-ajax.php file to handle/save html data.
Let us know if this helps.