Your production line can only produce one type of resource at a time. You can unlock additional resources and switch the production to the new resource. 
If the resource is switched the production line needs some time to adjust to the new working process
-> this is called the retooling time.

1. To speed up the retooling time navigate to the building overview of a production Building, then click on the "production line settings button":

2. Click on the "upgrade retooling time button" and wait for the upgrade to be finished.