[Freesurfer] problem converting .ima files

Hi, I am trying to convert .ima files, and my command seems to work fine for some series:

mri_convert -it siemens -ot nii 523-2-4.ima ../srs2.nii

but not others:

mri_convert -it siemens -ot nii 523-3-132.ima ../srs3.nii

These are series collected from the same scan session. The data were reportedly collected at MGH: Siemens Allegra, around the fall of 2001 (if that helps at all). I'm running freesurfer-Darwin-leopard-i686-stable-pub-v4.5.0

I don't receive any error messages. The problem seems to be similar to one previously described in the archives, although I was not able to find the final solution:

http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg00890.html

Any guidance would be appreciated.

Thanks, Scott