Had my first attempt at making coffee in an old fashioned percolator this weekend. Had three attempts and let it perk longer each time, each time the coffee was very weak. The percolator instructions said to put six tablespoons of coffee in the basket, I used Dunkin Donuts original blend, ground coarse. And filled with water to just below the basket. The last time I let it perk for 15 minutes, and it was no better. What is the secret?