Do you see any page-builder initialization errors?
Somewhere in your code you have something like:
HttpContext.Kentico().PageBuilder().Initialize(yourCustomType.DocumentID);
I've seen this type of behavior when you only retrieve Published pages, or have an issue with multiple cultures and you want to add or change widgets to a page that's not published yet.
If so check where you retrieve the Document and based on current state include unpublished pages, get the correct or combine cultures.