I've had the same problem for a while now. Replay buffer works in simple mode, but simple modes strips too much away for that to be considered a fix for me. Any advice on this one? Plenty of other users out there who have this working.
If you go into your output settings. You will see four tabs. Stream, Recording, Audio, and Replay buffer. Click replay buffer and set your duration. Then you should be able to click the box that automatically enables the buffer! Hope that helps! If it does stop by my stream! www.twitch.tv/tfcrimson