So usually saying "Fitness" when using Xbox Fitness works great, voice commands come up, although the timeout could be longer when trying to read through the workouts in the library. But I've had a problem three times now where it'll stop responding to "Fitness", no matter how many times or ways or distance from the Kinect I say it. The Kinect is working fine, because I can say "Xbox" and the green voice commands come right up. The only way to fix it is to go Home and then quit the Fitness app and restart it, then it works again.
Just a guess but it seems like a potential memory-management issue where if it's been running for a while it needs to be killed to work ok again. Just wondering if anyone else has experienced this or if Xbox Support is aware of this issue along with the rest (crashing at the end of workouts, bandwidth errors, etc) so it gets forwarded to the right group.