How to Fix 'Hey Siri' When Your iPhone Stops Responding to You
When Hey Siri was first introduced in iOS 8, it was a pretty lackluster feature since you had to be plugged into a power source in order to even use it. Since then, newer devices (iPhone 6s, 6s Plus, SE, 9.7-inch iPad Pro) don't need to plug in to have always-on Siri, which makes the feature actually worthwhile—when it works.
Hey Siri can develop a metaphorical hearing loss over time on all iPad and iPhone models, no matter if you're plugged into a power source or not. This makes it very frustrating to use, since you basically end up yelling HEY SIRI with zero response.
To fix this, Apple introduced the option to "retrain Hey Siri" in iOS 9. By making the voice activation feature relearn your voice, you can use Hey Siri just like you could before. Doing so is simple, but the option is tucked away, and you might have to perform it again the next time Hey Siri stops responding to your voice.
The last two will be other phrases that help it learn your voice. After the setup is done, simply tap the Done button on the "Hey Siri" Is Ready screen.
This whole process should take you about a minute and will make Hey Siri your useful digital companion once again. At least, until the next time she stops responding to your voice. When that happens, just repeat this process again.