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melty - i am not an expert on brisket, but i think that your cooking temps might be quite a bit to low. you might be drying it out faster than you're cooking it (think jerky).
suggestion - try to find a good balance between cooking time and desired internal temps + time for collagen/fat/connective breakdown. one variable that you control is temperature.
try your next briskt at around 250 degrees, maybe even 260, and see what happens.....