I had 12.5mm plasterboard placed over old plasterboard that was wallpapered. I didn’t remove the old wallpaper as the plasterer just dabbed and added the new plasterboard on top. I’ve since had this skimmed and painted. The noise from neighbours now seems louder than before and when tapping the plasterboard, it sounds hollow. Has something not been done correctly? Any advice please? The skirting boards haven’t been fitted yet. Thanks in advance