I agree with mucky that your binding needs to be full. there are two ways that I think about this---the first is that I sew a 1/4" seam from the edge of the top piece sewing binding on, THEN trim all thicknesses to 3/8" before turning to the back to handstitch down. Or sometimes I get impatient/stupid and trim first to 3/8" them sew binding on at 3/8". The whole idea to me is to have your binding feel full with no feeling of only the 2 thicknesses of binding fabric hanging out there together---needs to be thick to the edge of binding.