@the5thdimension advice given is often dependent of the theme that is used. The fist question you need to ask is: does it apply to the theme I'm using. If you supply a link to your site I can take a look.
You probably have a conflicting .post p further down in the stylesheet that overrides it. CSS resolves conflicts in 2 ways: 1 what is last in the document. 2 what is more specific.
example: you want to style this XHTML with CSS
<div id="container">
<div class="textbox">
<p>some text</p>
</div>
</div>
If the CSS looks like this:
p { color: blue;}
/* other css */
p { color: red;}
The text will be red because it's further down the page
If the CSS looks like this:
.textbox p { color: blue;}
/* other css */
p { color: red;}
The text will be blue because the rule is more specific.
If the CSS looks like this:
#container p { color: blue;}
/* other css */
.textbox p { color: red;}
The text will be blue because an ID is more specific than a class because the unique ID can only be used once in the same (X)HTML file.
Hope this helps a little