Line Breaks Being Removed

This issue seems to be an old one, but I have yet to come across a realistic solution.

I have two sites, Ultimate TinyMCE is installed and activated on both. Both WP 3.3.1, two different themes.

On one site I have no issues.

On the other site, when I edit a post using the visual editor, all line breaks get removed. I have to then disable the option in the user profile and go back and fix all the HTML.

I do NOT want to spend my time doing this! It’s tedious, takes a lot of time, as you all know.

I believe there must be some other way to use the visual editor without everything smashing together. (E.g., this doesn’t seem to be an issue on my other site.) I DO want to be able to go back and edit posts, enhancing text format and layout. I DO want to add an image and then see what it looks like immediately within the visual editor.

Have I missed something in regard to resolving this age-old issue? Is it Ultimate TinyMCE? Why is it happening on one site, and not the other?

Suggestions will be greatly appreciated.

-Tanya