Is it possible the negatives are overexposed or on the dense side? I find the Printfile sleeves fine for contacting as long as the negs are a 'normal' density. However, if the negs are on the heavy side, they seem to look a bit difused when contatcted through the bag, especially 35mm which is on a thicker base. Also, the edge of the frame and the rebate tends to bleed a bit. If so, I will usually re-contact those ones out the sleeves.
Are they in 'contact', ie. do you put a piece of glass over top?