Cannot modify an already defined ID selector?

I’m quite experienced with tableless layout pages, but I’m really getting lost now.

I made a site for my client some months ago, but now I have to modify the CSS due to some design changes. Unfortunately I can’t post some images of it, due to privacy issues, but the code basically looks like this:

<div id="menu">A bunch of menu items</div>
<div id="header"></div>
<div id="leftBar"></div>
<div id="wrapContent">
Content goes here
<div id="footer">Footer goes here</div>

Everything is positioned using float:left, except the menu, which is absolutely positioned on top of the header.

Since the menu has to be move up 100px, so I changed that. But guess what? I doesn’t change anything at all. I’ve tried with other ID selectors, changing background colors etc. Nothing happens.

It’s like I can’t modify anything or something. Has anyone had such an experience before? What can I do?

I’ve already emptied the cache and stuff, tested it on IE, FireFox and Opera, it’s all the same…

Hope someone can help and sorry for the long post!

Thanks in advance