I know they recently had an app update. You may want to check to see if you have the latest version.
Under Update Profile|Shopper Notifications, I must verify they are turned "on" and make sure to check the "distance" of alerts. For example: Right now I have my profile to only alert me if the opportunity is within 15 miles, so anything outside of that I wouldn't get a notification. And last but not least, I made sure to check the "settings" in my smartphone, to make sure notifications were "enabled" for the ISS app.
If all things have been checked and something still isn't coming through, you can reach out to ISS through their ticket system. There is always a super friendly support team member, there to help.
Happy Monday