I'd shorten your draw length by 1/2" to 1" and practice shooting your bow with your bow arm slightly bent. I believe your dl is too long and you're stretching to get it all the way back and this is why you're slapping your arm. By shortening your dl you won't stretch, you'll likely be able to bend the bow arm slightly and be able to completely avoid this from happening but this is just my $0.02, Hope it helps
