Plugging - Halftone areas or reverses begin to enlarge and fill in. Plugging is also referred to as "filling in".

Ink emulsification
Fountain solution is too acidic
Reduce amount of concentrate or switch to less acidic solution.
Fountain solution conductivity is too high
Remix fountain solution to lower conducitivity.
Storage gum is still on plate using incorrect plate cleaner
Clean plate with plate cleaner. Make sure you are using the appropriate acidic or alkaline cleaner recommended for your plates.
Burnishine recommends using our fountain solutions to combat this issue: Universal Fountain Solution, Universal SM Fountain Solution, Universal Fountain Solution - SM Plus, Ultrafount Fountain Solution, as well as our plate cleaners: Plate Sav-UR, Aqua Blue, Plate Care and Gold Miracle.