I need to make one of my activities called MyNoStatusBarActivity.java a full-screen activity.
I have added in the Manifest :
I think you just need to take the android:theme="@android:style/Theme.NoTitleBar.Fullscreen out of the mainfest, stick with code only call, it's always worked for me. Which test device are you using?
Also make sure these calls come before you call to setContentView(R.layout.xxx) like this
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
setContentView(R.layout.xxx);
Check all of your code for android:theme declarations, ie manifest and individual layouts xml files. as it sounds like declaring it at app level is working (as it's overriding any subsequent theme declarations) but there is something overriding your activity level request.
Try searching your whole project for "android:theme" and remove anything that you're not 100% sure you need, test it that way