Are you using the official SmartThings supplied ‘Virtual Switch’ device handler?
Would you mind sharing a screenshot of what you are seeing? If you are already subscribed to a particular attribute, then you’ll see a star on the device card in the main SharpTools for Android app and the option in the menu (when long-pressing on the device card) will change from “Subscribe to switch” to “Unsubscribe from switch”.
One thing to keep in mind is that newer Android versions have become really aggressive with battery management, so you’ll want to make sure you run through all the de-optimizations noted in this article (and the article linked within):
I’d also note that there are a few other approaches to this. For example, I tend to use Pushbullet (or Pushover) for pushing events down to my phone to trigger things. Both of those apps are 100% focused on push events, so they have some tricks up their sleeves to make sure events are reliably pushed to your phone even with all the recent battery restrictions Android has introduced.
Since you’re using CoRE, there’s a Pushbullet DTH and Pushover DTH that I believe would work for you.
Personally, I use the (free) SharpTools.io Rule Engine which has native Pushover and Pushbullet support to push events down to my devices to trigger tasks.
If you go the Pushover/Pushbullet route, you wouldn’t even need the SharpTools Tasker plugins as both Pushover and Pushbullet have native Tasker integrations. (Though if you’re into Tasker and SmartThings, I’m sure you’ll find the SharpTools Tasker plugins useful!)