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