Let's assume the system is in working order, with a verified ground and clean hooks, and that the work piece falls into the original design parameters of the system. The first thing I notice is the word "sometimes." This means that sometimes you do not have trouble getting coverage. Therefore, the process is out of control.
Picture this: You have a machine that drills 100,000 holes a day, and sometimes the holes become ragged and have huge burrs. Here, the drill bit is showing wear. Once the drill bit is changed, the quality comes back. The manufacturer should monitor the process and learn when to change the drill bit before rejects are produced. In a powder coating system, what wear items could cause your problem, and why only the top guns?