Bluetooth Low Energy startScan on Android 6.0 does not find devices

I struggled with the same issue. To fix it you have to enable "Location" (GPS) in the settings of the phone as well as request location permission in the app at runtime. Both need to be done for scanning to work properly.

To request the location permission put the following in a dialog or the likes:

yourActivity.requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, yourPermissionRequestCode);

and implement:

@Override
public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults){
    if(requestCode == yourPermissionRequestCode)
    {
        ... //Do something based on grantResults
    }
}

in yourActivity and handle whatever the user selects. You also need to do the following to turn on your device's location services:

Intent enableLocationIntent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
yourActivity.startActivityForResult(enableLocationIntent, yourServiceRequestCode);

You can check if the user turned on the location services by implementing:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data)
{
    if(requestCode == yourServiceRequestCode)
    {
        ...//Do whatever you need to
    }
}

in yourActivity. You can also manually turn on location (GPS) services by doing:

Enter phone settings -> Select "Location" -> Then turn it on

It should look like this in the Phone settings:

Or like this in the quick-settings drop down:

Once the user has enabled the permission and started location services then you should start scanning. I've noticed that if you are already scanning while you enable the permission/turn on the location service it will still not put anything in your onScanResults

I'm not sure if this is a bug or a "feature" for iBeacons/Bluetooth advertising (NOTE: advertising as in selling products not the technical Bluetooth advertising) to allow companies to see your location and direct you to where they want.

Hope this fixes your problem!

EDIT I meant to add: you only need this for SCANNING. Once you are connected to the BLE device you can shut off the location service on your phone and you will still be connected to your devices. However, you cannot then discover or connect to any new devices and all advertising devices will drop from the onScanResults

Like @Jacopo Tosi say you must ask for location permissions. You can read here how to do it. And you also must turn on Location. i don't know why the did it. but it's the only way i managed to make it work.