Ink fountain ball is too cold.
Confirm that ink fountain ball is above 70°F(21°C) to ensure adequate ink flow.
Ink fountain is at a low level.
Add ink to fountain ball. Added weight will allow ink to follow fountain ball better.
Ink is thixotropic and gels.
Frequently work ink in fountain, use ink agitators, or add a small amount of overprint vanish to induce flow.