It's working now.
I believe the crucial detail was this:
"Requires that you be drawing in +tlrc coordinates, or at least have a transformation from +orig → +tlrc computed in the current directory."
I was drawing on a copied dataset that was originally in larger functional voxels. Perhaps that was the problem. So although I had a transformation from +orig --> +tlrc computed in the directory, it still wasn't working. After I copied the structural file using Zero, Func, As Is, everything worked fine.
Although clearly not as good as drawing individual ROIs, the atlas regions are doing a decent job aligning with the brains I'm analyzing.
Thanks so much!
-Adam
