I never add references to my stylesheet in the skin files, DNN will do it for you with skin.css. If you're using a different filename perhaps you'll need to add a reference.

Also, if you're working in the skins directory directly, and add a skin.css file after uploading your skin you might have to reset your web app in order for DNN to see the CSS file and reference it appropriately.

Here's a rundown on how it works:

DNN will use skin.css if this file name exists in the current skins' directory.
If your skin name is "abracadabra", DNN will also look for "abracadabra.css" and use that as well as skin.css to render the page.
In this way you can keep styles common to a whole package of skins in the skin.css, and only styles that are particular to a certain skin in a separate css file.

So, you can have myskin.html and mydifferentskin.html, and package them up with skin.css, myskin.css and mydifferentskin.css - each skin pulling common styles from skin.css

How's that?