Yeah its happened to me my app successfully completed 12 testers for 14 days but i got an email exactly like this . its happening because the 12 testers are not testing the app daily thats why its happening . then i have uploaded my app in a app called closed test pro . i got 12 testers from this app free and the 12 testers are tested my app everyday . this app has daily reminders which helps users test the installed apps once per day for 14 days .