That doesn't necessarily mean that they don't add lift. They are just pitching the nose down, like changing the elevator trim. Whether the nose goes up or down depends on many things. Normally, the flaps by themselves will pitch the nose down, if the CG isn't too far aft of the 1/4 chord. Then, the increased downwash behind the wing will tend to pitch the nose up, when it hits the stabilizer. The two things work against one another. If the stabilizer is of small area or aspect ratio and/or the flaps are large, the nose will likely go down. If the opposite is true, the nose will go up. In each case, the flaps are still adding lift. You should be able to fly slower before stalling.