Please try this, it is working perfectly.
//mc is the instance of stage movieClip which has to be shown.
//myMask is instance of movieclip in which region movieClip mc is to be shown.
mc.mask = myMask;
//Keep movieClip mc at initial postion and myMask in center of the stage.
mc.x += 10;
Its good to ask about the problem and solution. I will explain you in detail what the problem is and how I solved it.
Actually my problem is I was trying to load data from URL and add the loaded data to a movieclip (I assumed this movieclip as appsHolder). I want to add Scrolling for this one. For that I created one class with masking, scrolling functionalities. In that class I created one movieclip called holder and add the previous appsHolder to holder by calling a funtion. Here I applied masking to holder. and also every time the data is loading, I create one new appsHolder. As I mentioned above I got a problem in masking.
I am trying to analyse my mistake and found that every time creating a new appsHolder generates a problem. Immediatley I removed that code and create appsHolder only once.
And also I want to mention another thing, masking applied for either holder or appsHolder doesn't represents difference.
I don't know whether it is logical or not. But finally the problem is solved.