Causes:

Low ink level in the fountain
Improper roller setting
Ink Emulsification
Ink backing away from the "fountain" roller
Roller Stripping
Contaminated ink fountain – dust, grit, etc.

Solutions:

Keep ink fountain full to maintain head pressure
Reset rollers
Decrease water or add heavy body varnish
Agitate ink in the fountain or increase ink flow
Deglaze the rollers
Remove ink, clean fountain, replace with fresh ink

Causes:

Too much impression
Too much ink being carried
Low viscosity ink may look mottled on papers with uneven holdout
On some stocks, transparent inks will show more mottle

Solutions:

Check Blanket Impression and plate to blanket pressure
Run a stronger ink to permit a thinner film
Higher viscosity ink will have less absorption and reduce mottle
Use a more Opaque ink