Follow the advice given so far.
If there is still a problem check the headphones the drummer is using, the units should be putting out plenty of volume. If he does not already he should have either IEM's or some cans which isolate, in nothing more than for the sake of his hearing.
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Always make sure you have plenty of gain when working with P16's I often have channels gained a little higher console side than I would otherwise as it gives the IEM's good solid signal.
With the P16 the most common problems I see are that:
1. Improper gain staging (one or more volume's all the way down)
2. Invertible someone has panned the entire master mix?! (Why!)
3. The limiter is pumping hard (remember that for the limiter being off, or down, is the opposite of all the other knobs on the p16)
Also, I frequently encounter church musicians who do not own IEMs. Which usually means they cannot hear themselves well as normal earbuds often have no fidelity, cannot handle the signal without clipping, or have a phone mic which lends to the cord not connecting correctly. If this a problem you face check out these inexpensive
IEMs with these
tips. These sound tremendously good for the price and anyone should be able to afford them, and perhaps you can get your church to buy a few pairs of house one's (we usually keep a pair or two with an organizer/divider box which has tips, this way the IEMs can be shared but individuals still have their own tips).
If you don't already, I would recommend that you know the P16 in and out. Know how to fix each problem (linked channels, gain etc) and figure out what people need for a mix, so you can help those less experienced individuals (for example a bassist probably needs drums and the WL, and not too much more else while a vocalist might just need the WL and other vocalist and minimal drums or bass).
Before the musicians arrive I always try to make sure all the units have the appropriate settings, and sometimes I make a mix for someone, sometimes with an unprofessional musician who has no desire to learn it easier for me to take 30 seconds on the front end.
A few minutes into practice, after things are working in your world and your gains are set and everything is sounding fairly cohesive go check that everyone has a good IEM mix. I usually walk the stage and tap my ear and questioningly give a thumbs up (they can give a nod or shake their head in reply, this does not interrupt practice). If something needs work I will wait until their next break to see what needs help, usually, it is on their end.
If you have musicians or singers who do not understand how to take to use the units do a quick 2 minute training sometimes saying "these are the first problems I find, if something doesn't sound right let me know and I will help you dial it in, let me know now don't wait for the whole service :-)" Overtime the continued education seems to be working, and some of the musicians are actually helping troubleshoot the other ones. :-)
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Also, how are your p16's routed? Especially for drums post-EQ and compression makes a huge difference in the ears. I run most other channels post also. However, this also means that you cannot do any insane chopping on the EQ, although I can usually get by with reasonably moderate stuff when the house needs it. Ideally, you should be getting good tone and signal from their end, and you should not need to fix it, but at least with one of the churches I work with that is simply not usually the case...
