Robocallers are constantly changing their phone numbers so we update the blacklist in the app every 15 minutes throughout the day. Normally there's nothing you need to do to keep the background updates happening but if they're not, here are a few things to check.
Don't force-quit Nomorobo. If you force-quit the app (by double-tapping the home button and swiping up on the app), then iOS prevents background updates from happening until you relaunch the app.
2. Make sure Background App Refresh is on. You can find this setting in iOS Settings > General > Background App Refresh.
3. After restarting your phone (e.g. after installing an iOS system update), you'll need to launch Nomorobo. Apple only allows apps to work in the background if they've been launched at least once after restarting.
4. Enable the option to share your contacts with the app to block spoofed local calls.
Assuming you've allowed Nomorobo to send notifications, the app will notify you if it hasn't been able to update in more than 24 hours. When that happens, launch the app and it will immediately update its blacklist.