An email can specify multiple body parts, text and HTML.
Sometimes the email bot might only be specifying an HTML body and no text body.
Within the HTML the JSON is escaped and, therefore, cannot easily be parsed.
Please double-check the bot implementation and verify it also attaches a text body with plain JSON in the email.
For test purposes you can add a new task to the beginning of your workflow with the fields Trigger email / Body Text and Trigger email / Body HTML to a user task to see the results or just log them within a script task.
You can also see the actual email that triggers the case, under case history, however the above method is easier to debug if you are sending JSON variables for example in the body of a email.
If you have any more issues please let us know the email address of the trigger you are using and dates/times you sent the emails and we will investigate.