The automaker plans additional downtime at the Chicago, Flat Rock, Kansas City and Kentucky Truck assembly plants this month and next, in addition to limited production at its Ohio assembly plant.

Snarled shipping lines are disrupting the movement of natural rubber, compounding a shortage that threatens to further disrupt vehicle production.

The industry has been confident that the strange problem would straighten itself before long. That now seems unlikely.

The automaker’s North American production was hammered harder than that of its rivals last week.