Have you uploaded any other type of attachment (say an image) to the same thread in the same forum using the same user, to rule out permissions or an unrelated upload issue? Have you tried a different browser or get a different member to try uploading it if you are sure your attachment settings are right? Possibly also try changing the upload limit higher as a test (I set mine at 100000), any effect?
Do you have any mods/plugins active that relate to attachments?
Does the file contain any odd characters in the filename, have you tried renaming then uploading, or uploading something basic like a text.doc file with just a short text in?
When you say it does not appear on the list, do you mean it goes through the motions of uploading and then just vanishes? Do you get an error message and have you checked your error logs?
As a workaround if you can upload other attachments you could always .zip (compress) the word document before uploading - does that work?
Does it work trying to attach them to a conversation rather than a post?
I know some of this stuff is trying to teach you to suck eggs, but it might help just to rule out the obvious first, and narrow down whether this is a server issue, a Xenforo issue, a browser issue, or a problem with the actual file.