Dots in midtones screened areas, that normally should appear separated on printing, bridge together causing a dirty aspect on printed product.

Main addressable cause is anilox volume too high for the selected plate linescreen, or, viceversa, plate linescreen too high for the available anilox volume. The ink, collected by the printing surface of the plate dots from the anilox, is squeezed when touching the substrate and the droplets bridge together creating larger inked areas.

Other secondary causes might involve too high viscosity of the ink, or unsuitable ink, dirty particles on plate due to substrate dust (paper), excessive impression.

Reduce anilox volume or use printing plates with lower linescreen.
Check ink viscosity and suitability for application, clean the plate from eventual dust from substrate (paper), adjust (reduce) impressions both anilox/plate and plate/substrate.