<span> elements are inline elements, as such layout properties such as width or margin don't work. You can fix that by either changing the <span> to a block element (such as <div>), or by using padding instead.
Note that making a span element a block element by adding display: block; is redundant, as a span is by definition a otherwise style-less inline element whereas div is an otherwise style-less block element. So the correct solution is to use a div instead of a block-span.