The simple reason is that your document content differs from the description in your post. The Find expression is case-sensitive, to match the lower-case "see n" mentioned three times in your original post. That isn't the same as the "See n" in your document. You can process both by changing:
.Text = "see n [0-9]{1,}"
to:
.Text = "[Ss]ee n [0-9]{1,}"

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]