I found that it was an issue with the rounded head of one of the screws preventing the drop target from "latching" to the lip that it rests on. (The drop target can move VERY SLIGHTLY off-center from time to time.) I replaced the round head screw with a countersunk one and haven't had any issues since. No washers needed, but each case may be different.
Hope it helps!
Post edited by MrDo: Clarification