French press problems

Hey all, so I recently got a French press and been using dark roasted beans (I'd say full city plus to just a tad beyond that probably) from Starbucks (Ik it's what I could afford unforutnatly) it is whole bean so I grind it myself and I have experimented with modifying the grind size, amount of coffee amount of water steep time, quality of water, all this fu stuff and still the coffee results in bitterness and some actually good flavour, but mostly bitterness whcih is disappointing, these beans I feel have a lot of potential actually but I just can't get it. Any help is appreciated. Thanks guys!