Make sure the Pop.in app's Push Notifications are turned on, as this will prevent the SMS from arriving on both iOS and Android. Check to see if your cell device blocks SMS from unknown contacts; if it does, the SMS received from the short number may be blocked. Close the app and reopen it. If it's still not working, you could try and get a
temp phone number UK, and use to get the code on it. I often use this feature and it saved me a lot of times.