It could be your primers. I noticed this year while firing caps in the dark, that winchesters were throwing a flash about 6 inches out the end of the barrel. I switche to Remington Premier primers and they were throwig a flash almost 2 feet out the end of the barrel. All I will shoot now are the premiers.
Something is either partially blocking your nipple or you got some bad primers one.
I have never had a delay with them.
Well unless I was using number 11 caps like a moron anyway.