These kinds of problems are usually due to a conflict between two different addons or with your theme code. I took a look at your site but unfortunately I cannot identify what the exact problem is. If you paid for the theme you're using, I would ask the theme author about the problem (submit a support request on the theme's marketplace page). If you built the theme yourself, you'll need to use Firebug or Web Inspector to figure out where the style conflicts are.

Best of luck,
Jordan

I found my problem.
I use Knacss (http://www.knacss.com), a css framework and there is this code :
/* switching box model for all elements */
* {
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}

This is the reason of my problem.

Thank you for the addon.
Have a nice day.