If you're not receiving push messages on your app, there can be a few causes, one of which is that you didn't grant permission for the app to send you push messages when you first installed it, and the other is that you disabled push messages at some point.
Here's how to get back on track
For iOS
STEP 1: Go to Settings - find your application on the list of Installed Apps
STEP 2: Tap on Notifications
STEP 3: Toggle to Allow
For Android
STEP 1: Go to your device settings and select Apps & Notifications
STEP 2: Tap Notifications
STEP 3: Find your application in the list
STEP 4: Toggle the Notifications ON