Try these steps in order. Most slowdowns and crashes are fixed by updating and restarting.
Right now, during service
Force close Nomni POS and reopen it.
If it froze mid-sale, check Held Orders and search Sales > Invoices before charging again. Check your EFTPOS terminal history so the customer isn't charged twice.
If the Master is frozen, restart the Master iPad. Slaves reconnect when it's back.
After closing
Update Nomni POS on every iPad to the latest version. All devices must be on the same version; a mix of versions is a common cause of loading loops and disappearing orders.
Update iPadOS on the Master to the latest version (Settings > General > Software Update).
Restart all iPads. Restarting daily helps prevent slowdowns.
Make sure the iPad has free storage (Settings > General > iPad Storage).
Check the network
All POS devices, printers and KDS should be on the same Wi-Fi network.
Turn off Private Wi-Fi Address on each iPad: Settings > Wi-Fi > tap the (i) next to your network.
See Fix Wi-Fi drops on your Nomni POS iPad if the connection keeps dropping.
If it keeps happening
Contact Nomni Support with: your Nomni POS version on the Master and each Slave, iPad model, when it started and whether it happens at peak times, how many Slaves are connected, your router, what is affected (POS, KDS, printer, QR ordering), and screenshots of any error with the time it happened.
How to update Nomni POS
Open the App Store on the iPad and search for Nomni POS.
Tap Update. If you see Open, it's already up to date.
Update the Master first, then every Slave, so all devices are on the same version.
Reopen Nomni POS and run Master Sync.
Tip: turn on automatic app updates in Settings > App Store, and update outside service hours.