What OS are you using? What version of Dreamweaver?
What extensions, including the version numbers are in stalled?
When you edit the design, are the CSS files open in Dreamweaver? Make sure they are closed. if they are open that can cause changed to be disregarded.
In your site, do you have the file check in / out setting enabled?
if so you will need to checkout the CSS files in order for changes to be saved.