I'm sorry about what happened. I agree with the others, without finding the deer and doing a post-mortem, it is hard to say what happened. Most likely you missed the second neck shot, because a neck shot with any bullet will usually drop the deer initially (even if they get back up later) from the shock of the bullet passage. However, it's hard to say what happened with the first shot.
It could be IMO one of 2 things:
1. Less than ideal shot placement
2. PB blowing up. If the PB had stayed together and fully penetrated the deer, even without expanding at all, it would have killed it with correct shot placement. If the PB fragged and didn't fully penetrate the deer, it might have had one lung shredded, but the other lung intact. A deer can go a long ways on 1 lung I've learned from a lot of bowhunting.
But without recovering the deer to find out what happened, it's all speculation...