I create my xmp files in Notepadd++.
One of my xmp files used SS&A Cup in the metadata and when trying to save the file from Dropbox using Save to Device on my iPad, Shuttersnitch would not open the Metadata Preset Options, instead it just opened Shuttersnitch without any prompts.
To workaround this I replaced & with & which appears to be the correct XML syntax for escaping ampersands.
Just in case anyone else has this issue.
:)