Okay, I give up. Why can't I get the sludge out of my coffee using my bodum thermal french press?

I'm using the coarsest grind I can buy at Starbucks. Do I need to buy a grinder for home and make it even coarser? I've read all the instructions and tips I can find. The plunger goes down (with some effort), but the coffee is invariably thick and not in a good way, to the point that there's a fine layer of silt in the bottom of the cup.