I would guess that the reason that you aren't detecting anything with the sample app is that the vision libraries weren't successfully downloaded to your device. This will happen if the device is in a "low storage" state. We recently updated the samples to check for this condition and provide feedback to the user. For example:
// Check for low storage. If there is low storage, the native library will not be
// downloaded, so detection will not become operational.
IntentFilter lowstorageFilter = new IntentFilter(Intent.ACTION_DEVICE_STORAGE_LOW);
boolean hasLowStorage = registerReceiver(null, lowstorageFilter) != null;
if (hasLowStorage) {
Toast.makeText(this, R.string.low_storage_error, Toast.LENGTH_LONG).show();
Log.w(TAG, getString(R.string.low_storage_error));
}
https://github.com/googlesamples/android-vision/blob/master/visionSamples/photo-demo/app/src/main/java/com/google/android/gms/samples/vision/face/photo/PhotoViewerActivity.java#L91
If you are encountering this, freeing up space on the device should do the trick.