live mixing--speech and music

Hi, i help mixing sound in a church. We are using a mackie mixer and as per their instructions, setting the preamp gain so that in solo mode each instrument reaches 0 DB. Thus mixing for music is going on fine. However we are unable to obtain such levels for speech even with the input preamp gain turned to 3 'o clock position. This is a major problem with preachers who are soft. Hence we are forced to turn up the power amp fader, and compensate by reducing our overall main mix fader during music and boosting it up to unity or more during speech. Doing this makes the levels useless.

Is this a typical problem? How is it solved generally..? Please help..