First, please confirm that you have the texting feature enabled. To check this, sign into your account on grasshopper.com and select Settings then Features then scroll to Grasshopper SMS. Is it on and enabled for your number? Next, check that you're signed in as an admin. Finally confirm that you're on the latest version of the iPhone app (4.1.6002 or above) or Android App (4.1.6100 or above).
Next, be sure that you've enabled push notifications for the Grasshopper app. You can verify this in the notifications section in the phone's settings page. Navigate to Settings then Notifications and find Grasshopper. Are notifications allowed? (if the toggle is green, then yes, they are allowed)
You can also check the settings within the Grasshopper app to make sure notifications are turned on for the Grasshopper number. Open the Grasshopper app and go to Settings. Scroll to and select Sync & Notification Setting. Ensure that all boxes are checked off for the extension(s) that you want to be notified for on this device.
For additional instructions on enabling notifications, please read this article.