If AutoFill does not work at a particular website or for part of a form, it is probably because the site author has turned it off. Financial sites do this most often (banks, brokerage houses, credit unions, and others).
Safari honors the AutoComplete="off" flag used by website authors. You can verify that the site author has turned off AutoFill by searching for this tag in the webpage's source file. From the View menu, choose View Source.