Please check the following:
Are you scanning with the camera launched by the MYAKU-PO! app?
Is your network connection stable? (Even if the Wi-Fi icon is displayed, it may not be connected to the internet)
Have you tried restarting your smartphone to see if the issue is resolved?
Have you tried clearing the QR code cache to see if the issue is resolved? (You can clear the QR code cache from your smartphone's camera or settings)
Is your smartphone's operating system updated to the latest version? (Please make sure to back up your data before attempting this)