Explanation: In the picture included there are two blueprints, Light-0 and Light-1. Light-1 is placed in the inventory. Light-0 is placed in a blueprintbook, and that book is also in your inventory. The picture shows the inventory on the left, and the blueprintbook contents on the right. The Quickbar shortcut to Light-1 works, the shortcut to Light-0 does not work. If you now drag the Light-0 blueprint directly into the inventory, (out of the blueprintbook), then the Light-0 shortcut works again.

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Factorio 0.17.8, Linux Mint 18.x