There are two things I can think of off hand that would cause a bounce off the shelf. The first would be too low of a nock point, causing contact between the rear of the arrow and the shelf when the arrow is shot. You could try a really high nock point, like 1” or more, and see if you still get a bounce. The second happens when a split finger shooter presses down on the nock with his index finger, causing a bow in the arrow at full draw, which then rebounds when the arrow is shot. This can be resolved by lowering the drawing elbow a tad, and being aware of any downward force exerted by the index finger when drawing the bow. (This could also be resolved by switching to 3 under, but I doubt you would like that solution 😊)