I can't init Vuforia in my own project

I can successfully run your sample code with my dev key, but when I try to integrate Vuforia SDK in my own project it fails to run. I try to init Vuforia in the InitVuforiaTask but Vuforia.init() returns 50 and then -1.

    private int mProgressValue = -1;     protected Boolean doInBackground(Void... params) {         synchronized (mLifecycleLock) {             Vuforia.setInitParameters(mActivity, mVuforiaFlags, «my dev key«);             do{                 mProgressValue = Vuforia.init();                 publishProgress(mProgressValue);             } while (!isCancelled() && mProgressValue >= 0                 && mProgressValue < 100);

            return (mProgressValue > 0);         }     }