White resedue?
Could be someone has done the old supa-glue as thread lock trick, in which case a bit of gentle manipulation should break the bond between PCB and riser (assuming the risers are metal?).
Does the thread "crack" loose when you loosen screws or is it tight all the way out. If it cracks then loosens its probably lock-tite (like AmigaPete said) or supa-glue, if the thread is grabby it could be anything from liquid nails to
