I would say the soil is a little too dry. The mix you used looks really good. I would use some tea, beer, wine, pee, whatever liquid you have that isn't water in a small ratio on the far outside. You have to be careful though as it can burn the roots if you use too much. My japanese maples glow certain times of the year, I have three boys and they know which trees to hit lol.

There is a bonsai forum...

Are you growing this tree in the house? If so that's your problem. Bristlecone Pines are temperate trees and cannot tolerate indoor conditions. For your first bonsai you picked on of the most demanding and difficult trees to work with. It also looks like your soil is garden soil or potting soil. This too is a major problem BCP's grow in almost pure dolomite at above nine thousand feet.