Its a funny thing, usually there is a reason cock feather in is working for you. The arrow is clearing better, or you are getting less, or a more beneficial bump on the riser. I have had this issue before, and usually it comes down to a torque of the bow, or an arrow not quite perfect spine, or a fletch job that just plain is in the way and kicks off the riser. I am not sure why, but I haven't had that make a positive difference in years now, but I do remember it has helped sometimes in the past.