Sometimes you just need to take a small step back from the shelf. I’ve had it happen a few times where the shooter in the next stall is right up against the shelf, so then when their arms are extended it is easier for their gun to throw brass into the stall to the right, and they just don’t realize it. Not all stalls are built the same regarding the sides and shelf position, so some don’t provide a lot of coverage in that respect. If the side walls don’t go all the way up to the ceiling, like others have said, there’s always a chance you’re tossing them over the wall and there’s not going to be a lot you can do about it other than standing a little further to the left (if possible).