Crafting in the Workshops is limited to the number of empty slots inside the building. When progressing in the game, you'll want to open up as many slots as possible so that you can set up several recipes to craft while you do other things.

To add slots to a Workshop, tap on the Workshop to open the crafting window. At the top of the screen you'll see a green button with a Gem and "+" (plus) sign. Tap the button to purchase the additional slot.

Note: The amount of Gems needed to unlock a slot will increase as the number of slots grows.