Why won't "Revert to Saved" load the version I choose?

I'm trying to revert to a previous version of a document I was working on, and I can get as far as the screen with all the versions to choose from, but when I choose a version and click "Restore," the window with the current document morphs like it's going to change, and the version I chose flashes briefly in it and then disappears, leaving me with the current working version (not the one I wanted!) again. Am I doing something wrong? Is there another way to access a previous version? I really need this copy!