Help! with JWP......

Hello everyone, this is my first time on this site. A little about me, I live in Pinole, CA and have been doing bonsai for about 7 years. I mainly have conifers. I am no expert and just got into conifers about 3 years ago. I have work on a lot shimpaku junipers and a few black pines. I have a few JWP that I got as started seeds that were already grafted. I recently purchase two JWP that were imported in 2008. One of them started good, but some of the new growth from the top has started to die off and dry up. The middle and bottom growth is doing great. I removed all the fertilizer cakes and have not fertilize it since. I been trying to figure what it could be and I need help to determine the problem. I doubt it is root rot as I am careful on watering, but I could be wrong. Attach are pictures of the tree and the dying area of it. I would greatly appreciate any suggestions

The rest of the tree looks fine. The top looks like it was injured by something external. Questions I have are these: can you detect insect damage in the area? When did the color of the needles start to change? a boring insect could have come out of winter hibernation and chewed its way out of the tree. Did something hit the top? with imported pines, wire can be used to thicken the bark, giving the tree an aged appearance. Maybe there's a piece of wire girdling the apical area? These are probing questions for you to do some detective work. if you lose that area of the tree, you can always turn it into a nice apical jin (trying to be positive here).