Digital BW, The Print

I am using a polaroid sprintscan 120. I have found that while in 
preview or scan mode the image looks fine (decent contrast and tone), 
yet when I open the tiff file it is often very light (looks over 
exposed) and requires considerable adjustment with the curves.  I am 
using the polacolor software (same happens with vuescan) both as a 
stand alone and as a plug-in with photoshop. I have the checkbox set 
to raw scan when saving the file.  Any ideas on what I may be doing 
wrong?  thanks in advance, David