Then today, several reports started cropping up online that the most recent update to Google Play Services actually disabled this option for some reason that know one really knows. (To check if your Google Play Services updated on your own device, you can check the color of Google Settings in your app drawer: if it’s green, it’s the old one; if it’s grey, it’s the new one.)
Since this feature could be essential if your phone gets lost or stolen, it definitely makes sense to go ahead and ensure this feature is re-enabled once more. To do this, simply navigate to your app drawer, select Google Settings > Android Device Manager > Allow remote lock and factory reset. You’ll then get a screen asking you to activate a new device administrator – select this option, and you should be good to go.
I’m not sure why this happened, but this is obviously quite a big security bug that, hopefully, Google won’t repeat ever again.