OP, you can reveal the name of the mystery shopping company as long as you do not tell us the name of the client. We can probably help you with some opinions and past experience if you provide more information.
1. What is the mystery shopping company?
2. How did you learn that your reports were rejected? What information were you given?
3. How long have you been mystery shopping?
4. Without revealing the client name, describe the shop. Many of us may have done the shop and we may be able to help you figure out how your reports did not meet the guidelines.