The SL880 has key velocity (how hard you press the key) mapping to personalize the feel of the controller. In most cases, you can select how hard you want to press the key before it starts sending high key velocities to your synth module to make the sounds louder. It also offers inverse velocities so when you press keys softly, the sound comes louder. Performing a hard reset is the most simple way to resolve this. Press +, -, and Enter at the same time while powering the unit up.