also be sure you have adjusted the DIGITAL settings (there is a more technical name)
its done by hitting TRUNK and then holding 9 when a digital voice is coming through
you will see the digital decode level at the top right hand corner, a lower number is better. hit upor down arrow to adjust and see if the level goes up or down
sorry for the vague details im doing this off memory
there should be more info int he manual on the adjusting the digital settings