it isn't any hidden attributes, it is based on their OVR...had a similar case a few years back...was bidding on a stud catcher who had very low durability which held his OVR down below 70...if i recall correctly, somewhere around 18M he kept rejecting my offer and i ended up losing him to another team...not sure exactly what the numbers are, but i'd venture to guess that any IFA with a "true" OVR over 70 doesn't have any limit on the amount that can be offered