I took me 90 mins to get in vs Spurs after the NFS failed at entrance K (it worked for most fans, but not for everyone). I queued for paper ticket for over an hour with hundreds of fans, and 20 minutes after KO, the stewards directed fans back to the turnstile (now working), and we got in. After the game, I phoned City and they said that they were investigating and collating details from fans including phone models etc. They said once they knew what the problem was they would be in touch with a solution/explanation. So far no one has been in touch. Meanwhile my ticket has appeared on my phone with no barcode once again, which means if NFS does not work, I might expect to miss around a 1/4 of the game again.
Digital ticketing should in theory be great for fans with mobile phones. It should make buying tickets, transferring tickets and entrance easier but City don't have the systems in place to make it work properly.