That’s their trademark. Essential to their branding is consistency from one store to the next, anywhere in the world. The challenge is that different beans taste, well different. Over-roasting disguises this variation so that no matter what Charbucks in the world you happen to wander into, you will be served the same crappy burnt product. (Not a fan.)
To the OP (a zombie, I’m aware), the question was answered early on. If the grind is too fine the coffee will taste bitter. If the grind is too coarse the coffee will taste weak. If the coffee is too weak, use more coffee. The rule of thumb is 16:1. One scoop (1/8 cup) of ground coffee for every two cups of brewed coffee. Adjust this up or down with practice.
One of the best coffee investments one can make is in a good grinder. A conical burr mill grinder is the nuts, producing a consistent even grind. An even grind is important because dust in your grind can lead to bitterness as noted earlier. The downside is that it’s hard to find a good one under $100. (Some will say $200.)
I use a press pot which I’ve found allows me to control all coffee brewing variables: grind and dosage, water temp and steeping time. My recipe: medium roasted Arabica, ground midway between medium and course, a 16:1 dosage (maybe a little stronger), water temp just off boil here at 2K feet and about a 3.5 minute steep.