I think this might be a problem with the theme I'm using (Nitobe). I'm also using the WYSIWYG rich text editor fckeditor.
When I make formatting changes to a post using fckeditor, such as bolding, italicizing, or more substantial changes, like adding a numbered list, it works fine. However, when I try to add line breaks, the changes are not reflected in the published version. I look at the html, and there's no <p> or <br /> tags there whatsover.
So, I might have some text that I edit like this in the editor:
"blah blah blah
foo foo foo"
When I save and view the node, it looks like this:
"blah blah blah
foo foo foo"
Then I go back to edit it again, and it looks like this:
"blah blah blah
foo foo foo"
Could it be that Nitobe and fckeditor are incompatible?
Solution: use an input filter without the line break converter.