Don't worry buddy, it happens all the time. But it could be many things.
There should be a form for this.
Flat or Packer
Trimmed and How?
Average Temp you smoked at
Type of smoker (uds, offset, static diverter, tuned diverter, what?)
How did you Prep it (Marinades Injections)
Fat Up or down
Any pit temp problems arise
DID YOU PEEK AT THE BRISKET APPROXIMATELY 40 TIMES EVERY HOUR TO LOOL AT IT...
How did you determine its doneness (Temp or Feel, Look, sound, yes sound)
What was in the Rub
any extra rub applied at certain times
Foil, no foil, pan, plastic wrap, butcher paper, pan, box, paper bag
Separate the Point?
Rest, no rest, how'd you rest.
We need details and then there's the rare case that I see from time to time where you THINK its tender, and you THINK it had good texture because you just compare it to everybody else's or some winner at a competition and the truth be told its not Tender and does not have good texture.
fOR EXAMPLE - I have had SEVERAL people call me with this problem over the years and popping it in foil or a pan in the OVEN at 300 for 2 hours or so seemed to fix it. Guess why. The person cooking it swore up and down it was "dry."
Popdaddy is Dead - 1933-2011 - Pitmaster T is a free agent