If you are cooking at 225 dome temperature, its probably too low. The first time I did a medium sized brisket, about 6 lbs, I put a probe on the grate. For the first 3-4 hours, with the dome at a steady 250, the grate was 190ish. After 9 hours, when I needed it for dinner, it was still too tough, and had an internal temperature of about 180.

The other thing I've found, and its already been mentioned, is that bigger pieces tend to give better results. It appears that the large mass of meat holds enough moisture so that the piece isn't dry before the collagen breaks down.