Doesn't 3dfim+ have a default polort value of 1? and there is default thresholding on, too, I think? From the program help:
[-polort pnum] pnum = degree of polynomial corresponding to the
baseline model (pnum = 0, 1, etc.)
(default: pnum = 1). Use -1 for no baseline model.
[-fim_thr p] p = fim internal mask threshold value (0 <= p <= 1)
to get rid of low intensity voxels.
(default: p = 0.0999), set p = 0.0 for no masking.
3dfim+ wasn't really designed for this application, so I would check the default behavior...
--pt