I had elixir working great running some minification and versioning of the files, but to deploy I had to change the public folder to public_html and now the manifest file isn't being written. I'm not sure why the public folder change would affect this, but it's the only thing I remember changing since it worked.
The combining and minification works fine, there's no errors, it just doesn't write the manifest file any more. Any ideas what I might have missed?
Thanks for the advice everyone, I solved it by adding a trailing slash to the build path similar to @sitesense suggested, i.e. 'public_html/' and it did it's thing in 'public_html/build'.