Some Pages may rely on the old version of the Builder which has been discontinued.
You can however migrate content from the old Builder to the new one in a few clicks. Here's how:
1. Find your Page in the Control Panel, Pages tab
2. Click Duplicate
3. Select 'Keep layout & content' in the popup
4. Your content will load in the new Builder
And here's a short guide to the new Builder's capabilities.
Remember, it will need to be reshared with its new URL.