The autocomplete attribute is new in HTML5.
Since you haven't mentioned the DOCTYPE, I think this is the only possible reason could be this. For further details check MDN's How to Turn Off Form Autocompletion.
Try adding the following HTML5 DOCTYPE.
FYI: In some browsers you may need to activate an autocomplete function for
this to work (Look under "Preferences" in the browser's menu).
You're using it in a correct way mentioning autocomplete attribute under form element. However you don't need it to be mentioned separately for input tag.
Update: Here you can find a list of solutions in autocomplete attribute and web documents using XHTML.
