In order to control the amplitude of the sound with keyboard velocity, you have to enable this by setting the velocity to control the envelope amount. A common mistake is to set the envelope amount high thinking the greater the amount the more velocity sensitivity you'll have. If you want more expression the envelope amount should be set low. If envelope amount is set high, that value becomes the minimum envelope amount from any key press, and keyboard velocity will only register higher if the keys are pressed truly hard.
Conversely, if envelope amount is set low then that value becomes the minimum, and a wider range of modulation by key velocity is possible. A lot of the factory programs have a high envelope amount for demonstration purposes and therefore don't immediately lend themselves to much amplitude modulation without some adjustments.