If you can't figure out what the problem is in your code, then you can simply validate your CSS and HTML and that will give you a general idea of the problem.
CSS validation:
http://jigsaw.w3.org/css-validator/
Markup (HTML/XHTML etc.) validation:
http://validator.w3.org/
It will tell you for example, that you didn't close a particular tag which can make your webiste look broken...
I hope this helps.