Your use of deprecated <center> tag may be the cause of your problem. That's been gone since HTML4.01 and should not be used anymore.
To center an element you need three things:
- a valid DocType
- an element with a width
- that elements right/left margins set to auto
You also float all divs left. I have a pet peeve about floating full width elements as floating is for putting two elements beside each other. I would recommend not using this bit -
You should validate both your markup and your CSS. See the links about validation in my signature line below.