This sounds like an issue a friend had with his pellet cooker. The fire comes from below and he cooked it fat side up. He said it was so dry he had to cut half the brisket away. In my experience put the fat cap between your fire and the brisket. If the meat isn't drying out and is just tuff, I would either lower the temp and let it ride out for a longer cook or let it rest for a couple hours and see if the muscles relax and become probe tender.
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