Try clicking on the non-responsive Shelly in the app, then at the very bottom on the cogwheel (settings) and again at the very bottom "Device Information". This will show you your Shelly's last IP address (e.g. 192.168.1.101). If you click on it, it will open your browser and try to access the Shelly directly. If a page opens, you get the minimal, cryptic interface of the Shelly in question, but you can do almost anything. If the page doesn't open, you'll have to do a Factory Reset (remove and restore power and within a minute press one or other of the buttons 5 times).
Good luck!

Hi, when I have that problem, I reboot the device by switching off the power, and restart the Wi-Fi system and it works again.