Some bows that came with elevated rests already installed really don't shoot very well off the shelf, because of the way they were tillered. Some do shoot okay off the shelf, so you just have to play with it. I would guess that if the shelf slopes downward, the bow was probably designed to be shot off the elevated rest. If the elevated rest is not too much higher than the shelf, it might help if you build up the shelf to the same height as the elevated rest. You might be able to do this with multiple pieces of leather glued together with Barge or something, tapered as suggested above to offset the downward slope of the shelf. Or, if the elevated rest is quite a bit higher than he shelf, by carving a block of wood to create a new shelf close to where the old elevated rest is.