sometimes this happens from time to time, where the "client" doesn't show up or game window, make sure the game window is up and not minimized, if that doesn't fix it then switch uh scene to see if that helps, for me I make scene for different games. If that doesn't work then close the window that has the drop box and then maybe restart obs, making sure that the WOW window is up and not minimized the window has to be present for obs to capture it. I have Diablo III and can capture it fine, also a helpful hint, once you do get it to capture you can go into edit then transform or where it say transform then go to stretch to screen you might not need to do it if you got a big window for it to capture like 1280x720 or whatever, but for me I'm usually at about 800x600 (in game) so that I can see the twitch chat.
I'm seriously considering making youtube videos to show people some of the ways to use this, and to show some of the problems I've encountered while using OBS, I'm noticing btw Jim that maybe wavtap or just audio capture seems to interfere with OBS, also using itunes while streaming and playing in game sounds seems to affect it too. I say this cause every time I've had a problem with OBS its cause the audio then ingame audio start to stutter or stop and then I look at OBS and its like dropping frames and that the CPU usage kinda bumps up.