The possible reasons:

1. Output grounding of high voltage isolator. 

2. There is short circuit in negative plate or positive plate inside the electric field. 

3. The dust is accumulated in negative plate seriously inside the electric field. 

Suggested solutions：

1. Inspect high voltage isolator, the output entering into the electric field. 

2. Inspect the electric field and dispose the part with short circuit. 

3. Enter into the electric field and clear up the accumulated dust.