Diff module not tracking fields that existed before module was installed
Did you read the README for the module and go through the module configuration? See CONFIGURATION section of the readme.


2) Field revisioning settings

   Global settings per field type can be found here:

   http://www.example.com/admin/config/content/diff/fields

   "Show field title" toggles field title visibility on the comparison page.

   "Markdown callback" is the callback used to render the field when viewing the
   page in the "Marked down" page view.

   "Line counter" is an optional. This shows the approximate line number where
   the change occurred. This is an approximate counter only.

   Other fields add additional settings here.


This Diff Issue Queue item describes a similar "No visible changes." problem encountered by another user.

From the issue it sounds like the diff comparison view, which is typically "Standard comparision preview" settings == "Full content" needs to be configured to show the fields you want, or be the correct view_mode for the content.