Probably the single biggest issue is perception--some churches only employ a drum shield because church members "perceive" drums are louder when they are in the open than behind a shield. This is simply a leadership strategy to reduce the number of complaints.I used to play in my church and I absolutely hated playing behind the plexiglass wall. I could never get anyone to understand that the "unwanted sound" actually bounces back at me. At least I had in-ears to minimize that issue.
What I don't understand is why church music is the only genre to insist the drummer play behind a shield, a box, or even worse, an electric kit.
I've been to many clubs over the years and witnessed countless jazz, blues, folk, rock, etc. acts play without any of that crap. The sound was always appropriate for the venue.
Why can't that happen in churches?
The second reason so many churches require a drum shield is because of the blind leading the blind: they see other churches doing it, so in their mind, that's got to be the only way to do it.
Other common issues may include poor room acoustics, inexperienced sound techs, and drummers who have not yet mastered playing to the room.
Once we got our PA fixed (improper installation and feed to the board), made the appropriate acoustical applications to the room (installed treatments and re-angled the sheetrock on three walls), cleaned up the feed on our subwoofer and provided hands-on training to our volunteer sound tech crew were we able to ditch the drum shield and have our drummers play to the room. It sounds awesome!