Why is the beaconmanager just finding a beacon? - eddystone

Question

My problem is that the beacon manager just finds a beacon.
My code is below;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//location permission
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, 1234);
}
button = (Button) findViewById(R.id.b1);
//button onClick
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
mBeaconManager = BeaconManager.getInstanceForApplication(getApplicationContext());
mBeaconManager.getBeaconParsers().add(new BeaconParser().
setBeaconLayout(BeaconParser.EDDYSTONE_UID_LAYOUT));
mBeaconManager.getBeaconParsers().add(new BeaconParser().
setBeaconLayout(BeaconParser.EDDYSTONE_TLM_LAYOUT));
mBeaconManager.getBeaconParsers().add(new BeaconParser().
setBeaconLayout(BeaconParser.EDDYSTONE_URL_LAYOUT));
mBeaconManager.bind(MainActivity.this);
}
});
}
#Override
public void onBeaconServiceConnect() {
ArrayList<Identifier> identifiers = new ArrayList<>();
identifiers.add(null);
Region region = new Region("all-beacons-region", identifiers);
try {
mBeaconManager.startRangingBeaconsInRegion(region);
} catch (RemoteException e) {
e.printStackTrace();
}
mBeaconManager.addRangeNotifier(this);
}
//display beacon size
#Override
public void didRangeBeaconsInRegion(Collection beacons, Region region) { Log.i(TAG,"" + beacons.size()); }
}
The result beacon.size 1. But there are three beacons.
What should I do to find more than one beacon? I'll be happy if you can help me.