Could be any number of things.
Was it tender? Cut with a fork tender?
What did you do about the fat?
Did you make a gravy or sauce to go with it?
Seasonings were what?
Did you slice it across the grain or with the grain...or somewhat of both?
Ah ok, so not going to be the simplest answer.
Store bought brisket piece that seemed to have the fat fairly trimmed off (I didn’t season/salt it-probably an error), homemade chicken stock, tin of peeled plum tomatoes, red wine, onions, garlic.
In my big pot, I sweat the onions in butter and olive oil, then seared the meat (I think I did this in the wrong order here) and set aside. I poured in a glass of wine, cooked away the alcohol, put my tin of tomatoes and the brisket in the pot, put all my chicken stock in (which caused the brisket to be fully submerged - this might have been another error). Covered with the pot lid, in the oven at the lowest setting (100 Celsius I assume), cooked until I had to turn the oven off (about 9 hours), let cool in oven over night and placed the whole pot in the fridge as I had to go to work. Tried the brisket today after work, it’s soft enough (could be softer) - I haven’t pulled the meat apart or cut it yet, I just peeled some off with my fingers for a taste, peels away easy, but when I taste it, it feels dry in the mouth.