For weeks? Try opening the style sheet itself (by entering its address into the browser's address bar) and pressing F5. If it still doesn't refresh, your problem lies elsewhere.
If you update a style sheet and want to make sure it gets refreshed in every visitor's cache, a very popular method to do that is to add a version number as a GET parameter. That way, the style sheet gets refreshed when necessary, but not more often than that.
<link rel="stylesheet" type="text/css" href="styles.css?version=51">
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…