Air near the high-voltage ionizer needles breaks down in to ions and high energy electrons. This mixture of ions and electrons is called the corona plasma. The ions collide with suspended particulate and confer the charge.
Check the needles for roughness, a side effect of the corona discharge is the growth of a white quartz layer. This is an unintended reaction with cyclomethicone, a silicone oil present in household products such as deodorant sprays and skin lotions. This reaction leaves deposits of solid silicon dioxide on the ionizing needles. These insulating formations can suppress the corona pattern and intensity.
See page 6 of the AccuClean Service Manual for cleaning the field charger assembly. When installing this assembly note the air flow direction that is stamped in to the plastic frame.