OK, this is probably obvious to the gurus, but it's proving to be maddening to me. My "sold out" button is showing up lower than the "buy now" buttons it replaces.

Why would this be? I'm using stock button images and wouldn't know how to mess with their positioning anyway, so I really don't think it's something I've done. The only difference between the four items is quantity in stock; the two on the right have 0, the two on the left have 1. Any ideas how I can adjust this?