My thumbnails arn’t rolling over

Below is the css code I’m using to try and create a roll-over thumbnail. For some reason the background-position changes aren’t taking effect on screen.

.thumb {
	background: #222 url(images/thumb.png);
	display: block;
	height: 150px;
	margin: 0 10px 10px 0;
	overflow: hidden;
	position: relative;
	width: 235px;
}

.thumb:hover {
	background: #6894C2;
	background-position: 0px 150px;
}