Setting monitor levels during practice is crucial to the success of the band. A problem that can present itself, once the congregation starts singing along, is that the monitor levels needs adjusting. I've seen this at professional concerts where the musician jumps up a few times or gives a thumbs up or down signal to the sound tech. In the church arena, the less experienced musicians are focused more on playing correctly than having the right monitor levels. And it's easy to tell when their monitor levels are off; they sound bad.
A singer who is off-key or a guitarist who can't keep time with another guitarist are just a few examples of signs of a monitor mix gone bad. When I first started running sound, I assumed it was an indication of a talent problem. Then I discovered how I had the power to change their performance.
When the guitarist can't keep time, boost the monitor level of the instrument in which they are trying to keep time. Or, if that affects the stage volume, turn down the less-essential sounds in their mix. You'll know when the level is right when suddenly they are playing in time. You might need to boost their instrument level as well.
Off-key singers…boost the rhythm instrument which is setting the key. This could be a rhythm guitar or even a keyboard/piano depending on the sound arrangement.
Just because a band member is poorly performing doesn't mean they are poor performers. It might just mean they can't hear what they need to hear.
Question: How Does The Band Communicate Monitor Needs to You?