Question: "Hey Siri" immediately closes
i had been having this problem, and saw that many others had it as well. All the posts were too old to answer, but I found the solution. If you have Shazam, force quit it and Hey Siri will work again. I think it is because Shazam also listens in the background for music to match, so it cancels out "Hey Siri."
iPhone 6s Plus, iOS 11.2.5