It would really help to see the site online, but I tend to agree with Sam that you are probably missing the "display: block" attribute.
A suggestion though. Don't use Photoshop for website mockups. It's a poor tool to use for it. If you are going with Adobe, Fireworks is much better and it will make your wireframing much faster. The slice tool is more flexible which will allow you export chunks of your image directly into HTML that you can insert in the page.
Another tool you might look into Axure RP. This is actually an application dedicated for rapid prototyping and wireframing.