Normally, the entire coffee powder is evenly extracted even though the water always drips onto the same point. The coffee dreg should be entirely moist after the end of the brewing process. The coffee powder absorbs the water evenly and does not release the absorbed water until enough water has dripped in.
Sometimes it can happen that the drops pass very quickly through the coffee powder and the coffee does not become evenly moist. This results in a rather weak cold brew. This problem mainly occurs when the coffee is less absorbent and therefore prevents the water from spreading evenly through the entire powder. Low absorbency is a results off the coffee or grinding being slightly old, the coffee being very coarsely grounded, or lastly if you do not use enough coffee powder.
In this case, you have two options: Before you set the valve, you can drip water onto the entire surface of the coffee powder once. If the entire surface is moist, the drops are being distributed evenly. Alternatively, you can put a paper filter on the coffee powder so that each water drop is spread more evenly. Hereby, coffee or tea filters cut down to the right size are suitable.