What are you trying to add and how are you adding it?
Are you using any plugins that help you manage your content? Such as using shortcodes, javascript, etc.
For custom HTML use the HTML view. If you are adding JavaScript it will get removed for security reasons. If you use HTML view I suggest never opening the page again in Visual mode as you may lose custom HTML.
If your page has lots of types of content and many edits over time, sometimes it is better to go into HTML mode, clear it out, and start over clean as the WYSIWYG becomes a mess. Or clean it up yourself if you can edit HTML.
