Pot Roast

I made pot roast last night.
4.75 lb roast, cooked at 325 for about 3.5 hours. It came out sorta tough or rubbery.
I cooked it in a roasting pan, with about 1.5" of beef stock and one beer of liquid, lightly covered with foil.

The vegetables tasted good as did the broth (I pulled some liquid aside and mixed in flour and incorporated back and brought to boil)
It was a thick roast and I almost cut it in half before I seared it and put it in the roasting pan.
Did I just under cook it as in not enough time braising? The internal temp was about 170.