Help with CS2 History brush

[heart2heartmel]

I've never used the history brush before and have seen many other photographers use it quite a bit when editing. I've tried using it and sometimes I get it to work and sometimes it doesn't. There's gotta be something I'm doing wrong or not doing. Definitely user error. For instance, I was working on an image, made a few adjustment layers, then added a gaussian blur. I clicked on the history brush to go in and bring back the eyes, mouth, etc. and I just get a circle with a line through it....not allowing me to use the history brush. Do I have to flatten the image every time first before using it? When I've watched tutorials, they never mention flattening first, and it doesn't look like they're going through the step of flattening. ???