I have been having problems with my line breaks disappearing if I go back in and edit a post using the Visual tab. One solution I discovered during my research involves installing the TinyMCE Advanced plugin. This website explains the problem: http://rubayathasan.com/wordpress/plugin/enabling-line-break-in-wordpress/#comment-27261
Be sure to check the option box at the bottom of the Settings->TinyMCE Advanced panel to enable line breaks.
Let me know if this works for you or if you have found a better solution!