You have been testing WhatsApp Application for past few months and of latelywhile executing the test cases, you are not able to identify any issues in theapplication. What might be the reason and using which Testing principal thiscould have been handled.

There can be multiple reasons to it, to consider a few are:

a) You might not have updated the test cases as per the change in

requirements.

b) Change in OS [ Android -> Lollypop to marshmallow], [iOS 8 ->9], so if
we keep running the same set of scenarios and test data over and over
again it will turn out to be useless.  

This could have been handled effectively under ‘’The Pesticide Paradox”
principal which states if you keep running the same set of tests over and  over again, chances are no more new defects will be discovered by  those test cases.