This is a problem with multiple Sainsburys. I don't use their wifi as I'm not on O2 so don't have a password.Tried that...
The Wi-fi in Sainsburys can barely cope with the scanners let along the phone app traffic as well and the store is a giant faraday cage with no phone signal.
In one store my phone signal is too weak to use the app, and in another it sometimes hangs for a while with the spinning wheel and then the item you've scanned can appear in the basket in the app multiple times. I've never had this problem with their own handsets, although not all stores have them