Timbo, welcome to the group.
my first thought is not enough heat, but 7 hours at 220 and 3 more at 200 should do it assuming you are measuring temp on the same level as the brisket, and that your thermometer is accurate.
next is how was it sliced? you want to slice against the grain, not as easy as it sounds, cuz the grain on a brisket will make many twist and turns. Ya gotta reorient it every now and again make sure you got it the right way.
I am sure others will chime in with there thoughts too.
good luck and welcome aboard.
CB Bandera w/nameplate mod, 20 gallon barrel pit Weber Gold Kettle, and WSM's.