A bit hard to say the cause unless I can see a link. Normally, a parent page will display ALL child pages (folders module). It could be one of multiple reasons, possibly applied by mistake:
- "folders" module for the parent page is hidden?
- "folders" module for the parent page is remove from page items?
- You have set a "link" for the parent page that goes to an empty page?
- Parent page is assigned a different "assets" folder than itself?
I can only diagnose this with a link.