You have some nice bark on the BE's, but no bark on the slices? I would put fewer BE's in the box - like one line only in the front or in the back. Your slices look a little thick, which unless your brisket is falling apart overdone, is going to make it come across tougher and make it more chewy. I would slice them a little thinner, and put 8-9 slices in. You need to work on the symmetry when you place the slices in the box - the slices in the rear of the box are way out of alignment with the rest.
The judge would have done you bigger favor to let you know by checking a couple of boxes why they gave you the score they did. A few of them really didn't like your taste, so the next time you cook, really evaluate your beefy flavor(injection/foiling liquid), smoke (creosote/bitter?), salt, and sauce. You might try and wrangle a judge after they are done judging the contest to see if they will come over to your tent and tell you what they think of the brisket, or ask another team to do the same.
Also remember that your Costco Prime is going up against the top teams cooking SRF Black/Gold and the vaunted A9's, so you need to get A LOT of flavor out of those Primes, and you need to cook them perfectly. I think the primes are easier to get good BE's out of, but they are not as nice on the slices.