I want to obtain tracks from an image in the structural space. Hence I used the registration option in the FDT gui to obtain diff2standard_warp.nii, diff2standar.mat, diff2str.mat ,.. etc as taught in the practical. I then used PROBTACKX and checked the 'Seed space is not diffusion' box to obtain tracts on the structural image.

It worked on the first image and I was able to view the tracts overlaid on the structural image. However it did not work on the second image though I did the same thing. 

What could be the problem?

Thanks.