Negative Key is not overRiding With back on Device

In my Qt symbian app, I am over riding the negative exit button with back to come back on main screen and then again over ride it with exit to close the app, my app is working fine on emulator but when I test it on device it shows exit button instead of back, some time it shows back also but if I go to the same page twice then again it start showing exit button, frustrating part is that app is working fine on Qt emulator but not on device. Does somebody knows whats the problem is. I am using

back->setSoftKeyRole(QAction::NegativeSoftKey);
this->addAction(back);

to over ride the exit button before loading the screen and

back->setSoftKeyRole(QAction::NegativeSoftKey);
this->removeAction(back);

to removing back button when coming back to mainWindow. I am using Stack widgets in my application