You shouldn't see much difference in point of impact for 13fps if anything at all unless shooting 50 yards plus. Did it group ok before you sold it ? Try giving the barrel a good clean and see how it shoots then. I've come across this once before, and funny enough that was with a HW97 MK1, is this a MK1 ? No matter what I did it wouldn't group, so broke it for spares, put it down to a bad barrel. Let us know how you get on.
John