User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-71.120.219.7-20150720174646

I've read the life 3 summaries, and I've come up with a theory as to why Issei developed much slower then his predecessors, it was stated that Issei's parents were having trouble having a kid, then Issei was born, I think it was because the boosted gear bonded with him and gave him the strength to live, which is why it didn't activate when Raynare attacked him, which I always considered strange as a life or death situation like that would have been the perfect trigger, because his body couldn't handle it at the time, it was only after getting a physically stronger body and a whole lot of training that Issei was even able to use his sacred gear.