Brisket bottom is so HARD. Can't cut through it.

Every time I've cooked a brisket, the bottom of the brisket has ALWAYS turned into really hard bark. It's so tough and I can barely cut through it. The brisket shreds before I can cleanly cut through the bottom of it.

I've cooked on an offset smoker, green mountain grill. I've tried wrapping it in foil, butcher paper. Even putting it on top of a baking sheet.

Does anyone know why this is happening? What can I try to prevent this from happening? What's the secret!?