Hi, Dasherwood. I have read that uploading a word doc is sometimes problematic, especially if is is not a version that has been 'cleaned' of Word-specific tags, fonts, etc. that mess up Amazon's conversion to a mobi file format.
You might find some helpful alternative approaches (even if your other ebooks are working) at
http://notjohnkdp.blogspot.com/2015/07/all-about-e-publishing.html I used to write using Word then copy and paste plain text to an html document in Dreamweaver, especially since my books contain images but now, after reading notJohn's blog, I still take the text from the Word-MSS and tweak things by creating an EPUB using the free Kindle Previewer and uploading that to replace the html-images-zip file.
Check out his blog. Hope it helps.
AQ