Your production lines define how fast you can produce goods. Each production line has a product storage that defines how many goods can be produced. If the limit is reached, the production pauses until new space is created by removing goods or by upgrading the product storage.

1. To upgrade the storage navigate to the building overview of a production Building, then click on the production line settings button:

2. Click on the "upgrade product storage button", pay the costs and wait until the upgrade is finished.