I don't know why this bug exists, but if you use Services (I have it on for LastPass, for example), it messes with S Voice. You have to basically say the wake-up command twice for the microphone to become active without tapping it manually. I've tested this and as soon as I turn off all the Services under Accessibility, it works perfectly. Also, for any of you who have the talking calculator, this is also apparently caused by using any of the services. This is an extremely annoying bug because LastPass service is really useful, but I also wanted to use S Voice handsfree.