Rollers are improperly or inadequately cleaned.
Thoroughly clean up with a good quality roller glaze remover. (To remove glaze, apply hot water to rollers after removing ink. This will remove gum glaze that is not soluble in standard wash-up solvents.)
Rollers are improperly set; rollers are swollen or distorted.
Adjust roller settings or check roller surface condition.
Gum arabic and gum substitutes are in fountain solution; wetting agents have been used indiscriminately.
Control use of additives and use proven fountain solution.