Hey Siri doesn't work on iPhone 5S while plugged in.
My iPhone 5S has iOS 8.0. I plugged it in to my iMac (a late 2012 model), made sure Siri was on, and made sure Hey Siri was allowed in settings. No matter what I do, though, Siri won't respond to Hey Siri. I've restarted my iPhone a couple times, disabled and re-enabled Siri, and checked all settings which might be relevant. Still, when I say "Hey Siri", I get nothing. I try it with the screen locked, unlocked, in various apps, and I still get nothing. So far as I can tell, nobody else has this problem.
It's not worth it to me to reset to factory settings and restore from backup to get Hey Siri functionality. Any suggestions other than that? Something I might've overlooked?
iPhone 5s, iOS 8, 16GB