Google shows 404 for external file on cached page

I have a menu that uses Ajax to load links in a remote file. I use window.location.hostname to reference that file, since it is called for from different directories on my site.

I've been having ranking issues with Google, and in researching this, Google's cache of pages with this menu says it can't find the file and is showing a 404.

I'm using "http://"+window.location.hostname+"/directory/file.htm" and Google's cache says /directory/file.htm doesn't exist.

Seems to work fine on all browsers, what do I need to do so that Google can locate this file?